Issue with variables on upgrading from D7

When I try to upgrade from D7, I get the following error messages:

    Warning: unserialize() expects parameter 1 to be string, object given in system_update_1018() (line 2058 of /usr/www/users/adamg/uhub-backdrop/core/modules/system/system.install).
   Notice: Trying to access array offset on value of type bool in system_update_1018() (line 2059 of /usr/www/users/adamg/uhub-backdrop/core/modules/system/system.install).
   Notice: Trying to access array offset on value of type bool in system_update_1018() (line 2059 of /usr/www/users/adamg/uhub-backdrop/core/modules/system/system.install).

No other error messages come up and I can log into my new site OK, but there are some problems,  some minor (the admin toolbar doesn't fully appear and disappears completely on some pages), some more important (Leaflet in Views doesn't work, when it does on a fresh Backdrop install).

Any suggestions for tracking down where I can find the offending items in the D7 variables tables? I'm not surprised there's some kind of problem, when I started looking there were variables dating back to D4.7 (anybody remember flexinode?) - yeah, it's an old site. I've pruned everything I thought might be an issue, but am obviously missing something.

Thanks!