Lightroom Classic 10.1.1 won't start

[mochald]

Lightroom Classic, when trying to start, displays a window with the text "?: 0: attempt to index a nil value", reinstallation does not help, trying to start with a new (clean/empty) .lrcat file the same. Any attempt to do anything ends up with this message, and LR closes.

[johnrellis]

1. Try resetting LR's preferences: 
https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/

LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior. That article explains how to restore the old preferences if resetting doesn't help.

2. Roll back to LR 10.0:

https://www.lightroomqueen.com/roll-back-update-previous-classic/

[AxelMatt]

It could be an corrupt catalog -> A corrupt catalog causes an error message saying "attempt to index a nil value" (lightroom-plugins.c...

Try to open an older version of your catalog from the backup.

It can be also a plugin the issue. Try to deactivate your installed plugins and check again.