I managed to get past this by just changing the link, and updated to 1.30.0 successfully, but for some reason it generated the link incorrectly.
After starting the update process, and on the Backup step clicking "Skip Backup", the link generated was:
https://domain.com/core/update.php&op=selection&token=MyToken
rather than
https://domain.com/core/update.php?op=selection&token=MyToken
(& rather than ? before op=)
Comments
I just installed a Backdrop CMS site with version 1.27.0. I then used the user interface to download and update Backdrop CMS to 1.30.0.
After starting the update process, and on the Backup step clicking "Skip Backup", the link was correct.
Did you use use Backdrop to update itself or did you do a manual update (download and replace the core directory manually)?
UPDATE:
I also just ran the update manually, also without running into the same error that you did.
If there is anything specific to your site that might help us recreate this problem, please let us now.
If anyone else experiences this, they should try to generate some reliable steps to reproduce the problem.
Thanks for reporting your experience and how you got around it.