In my opinion, I suspect that there might have been some things deleted related to the OOTB extension. Apart from this, have you deleted anything related to the assistedservicestorefront extension?

"When examining the log file, I see that the error is generally originating from core files, and in this case, it could also be due to extensions. I suspect there might be a missing extension or a non-functioning extension. Have you tried rebuilding ...

Can you share me a screenshot of the first and last place of the error?

can you try delete addSeries value in DB if you not found addSeries valu can delete whit :-2146827850,"stack" number.

Can't you create the page you need with the wcms structure?