I love layouts. Once you get your head around how they work. But This is an annoying issue I am having frequently:
The "Main page content" block has incorrectly been positioned multiple times. This may indicate an incorrectly saved layout configuration.

This occurs after editing a block in my default admin layout, Update Block, then it throws this error. Save Layout throws the same error.
Workaround: Click Configure Layout, click Save Layout there, then it's happy...until next time.
Comments
Hmm, I've never seen that one before. I wonder if the admin default layout definition is somehow corrupted. This error should only happen if there is a block with the same uuid as another. Perhaps export and share the layout after saved?
I'm wondering also if perhaps a layout-related contrib module is messing things up? What contribs are you using?
Thanks mate, here is the layout:
{ "_config_name": "layout.layout.admin_default", "path": "", "name": "admin_default", "title": "Default Administrative Layout", "description": null, "renderer_name": "standard", "module": "layout", "weight": 1, "storage": 2, "layout_template": "vhb_ui", "disabled": false, "settings": { "title": "", "title_display": "default", "title_block": null, "alternative_paths": [ "org_table", "heart/my_openings", "heart/vhb-team", "heart/vhb-board", "heart/draft_faqs", "heart/wanting_tohelp_orgs", "faqs-ui", "heart/draft_insights", "heart/draft_openings", "heart/see_draft_orgs", "heart/unassigned_orgs", "testimonials-ui", "heart/openings_this_week", "heart/volunteers", "heart/roles_orgs_vols_contact", "team_vols", "heart/see_edu_orgs", "heart/seeall_emergency_orgs", "heart/seeallopenings_assigned", "heart/see_biz_orgs", "heart/see_gov_orgs", "heart/seeallorgs", "heart/assigned_orgs", "heart/assigned_orgs_total", "heart/archived_orgs", "heart/org_contacts", "heart/registrations", "heart/new_content", "heart/seeallopenings" ], "ancestor_matching": 0 }, "positions": { "0ba115db-f0be-44e3-a800-f5ca30df1c57--0": [ "eba5b724-6fc8-4a8d-9f19-25fccf87a3ee" ], "3f04167d-6c63-4a27-8f03-3cdc2e80222a--0": [], "ec4e937a-57e4-4afc-af00-004773eeefd1--0": [ "55f542fe-0763-453b-ba3e-62677ea559e5" ], "97bd4d56-184a-4e5a-8004-4b9df29f67d5--0": [ "ea437463-72dd-4038-a001-1d21b4d23033", "default", "52cd05d0-68ff-44da-b305-5f049b41f8e6", "752e5650-0e76-4cf2-a100-cbf9b754dee1" ], "f7248412-286b-4738-ad03-03597a14fd65--0": [], "f7248412-286b-4738-ad03-03597a14fd65--1": [], "f7248412-286b-4738-ad03-03597a14fd65--2": [], "title": [], "": [ "default" ] }, "contexts": [], "relationships": [], "content": { "eba5b724-6fc8-4a8d-9f19-25fccf87a3ee": { "plugin": "system:page_components:title_combo", "data": { "status": 1, "module": "system", "delta": "page_components", "settings": { "title_display": "none", "title": "", "style": "default", "block_settings": [], "contexts": [], "title_tag": "h2", "title_classes": "page-title", "tab_type": "both", "admin_label": "", "admin_description": "" }, "uuid": "eba5b724-6fc8-4a8d-9f19-25fccf87a3ee", "style": { "plugin": "default", "data": { "settings": { "classes": "" } } } } }, "55f542fe-0763-453b-ba3e-62677ea559e5": { "plugin": "system:breadcrumb", "data": { "status": 0, "module": "system", "delta": "breadcrumb", "settings": { "title_display": "default", "title": "", "style": "default", "block_settings": { "current": 1 }, "contexts": [], "admin_label": "", "admin_description": "" }, "uuid": "55f542fe-0763-453b-ba3e-62677ea559e5", "style": { "plugin": "default", "data": { "settings": { "classes": "breadcrumbs" } } } } }, "ea437463-72dd-4038-a001-1d21b4d23033": { "plugin": "search:form", "data": { "status": 0, "module": "search", "delta": "form", "settings": { "title_display": "none", "title": "", "style": "default", "block_settings": { "search": { "label": "", "label_hide": 0, "placeholder": "", "button_text": "Search" } }, "contexts": [], "admin_label": "", "admin_description": "" }, "uuid": "ea437463-72dd-4038-a001-1d21b4d23033", "style": { "plugin": "default", "data": { "settings": { "classes": "" } } } } }, "default": { "plugin": "system:main", "data": { "status": 1, "module": "system", "delta": "main", "settings": { "title_display": "default", "title": "", "style": "default", "block_settings": [], "contexts": [] }, "uuid": "default", "style": { "plugin": "default", "data": { "settings": { "classes": "" } } } } }, "52cd05d0-68ff-44da-b305-5f049b41f8e6": { "plugin": "views:volunteers-selected_org_contacts_block", "data": { "status": 1, "module": "views", "delta": "volunteers-selected_org_contacts_block", "settings": { "link_to_view": 0, "more_link": false, "use_pager": true, "pager_id": "0", "items_per_page": "10", "offset": 0, "path": "heart/org_contacts", "fields_override": null, "title_display": "default", "title": "", "style": "default", "block_settings": [], "contexts": [], "allowed": [], "path_override": false, "exposed": [], "admin_label": "", "admin_description": "" }, "uuid": "52cd05d0-68ff-44da-b305-5f049b41f8e6", "style": { "plugin": "default", "data": { "settings": { "classes": "" } } }, "conditions": [ { "plugin": "user_role", "data": { "settings": { "negate": false, "roles": [ "editor", "administrator", "manager" ], "load_condition_nojs": "Load condition" } } }, { "plugin": "path", "data": { "settings": { "visibility_setting": "1", "paths": "node/*/edit\r\nnode/add/organisation", "load_condition_nojs": "Load condition" } } } ] } }, "752e5650-0e76-4cf2-a100-cbf9b754dee1": { "plugin": "block:add_org_contact", "data": { "status": 0, "module": "block", "delta": "add_org_contact", "settings": { "title_display": "default", "style": "default", "block_settings": { "delta": "add_org_contact", "description": "adds the org contact after the org is created", "reusable": true }, "contexts": [], "admin_label": "add_org_contact", "admin_description": "adds the org contact after the org is created", "title": "" }, "uuid": "752e5650-0e76-4cf2-a100-cbf9b754dee1", "style": { "plugin": "default", "data": { "settings": { "classes": "" } } }, "conditions": [ { "plugin": "path", "data": { "settings": { "visibility_setting": "1", "paths": "node/add/organisation", "load_condition_nojs": "Load condition" } } }, { "plugin": "user_role", "data": { "settings": { "negate": false, "roles": [ "editor", "administrator", "manager" ], "load_condition_nojs": "Load condition" } } } ] } } } }Possible culprits?
Something definitely looks wrong in the "positions" subarray. Is this using a custom flexible layout template that you created? The weirdest part is the fact that there is an empty key under position, and the "default" content appears twice, under, "97bd4d56-184a-4e5a-8004-4b9df29f67d5--0" and under the empty key.
I'm not sure how this may have happened. I would try the following: