Some day Istarted the game as usual and found that repeated left-click actions don't work anymore. I press LMB somewhere near my character and move it without releasing. My character runs to the first point clicked and stops. Shift + LMB pressed produces only one attack.
The game becomes unplayable.

I didn't mess up with game files, no manual editing etc. Except for backup/restore saved games, sometimes while the game is running.

Any help appreciated.

I would go ahead and check the settings anyway. I've never had this happen with TL but other games sometimes do weird shit where I'll turn it on and for some reason the HUD will be turned off or something. Once, for reasons beyond my understanding, I started a racing game and it automatically put me in the cockpit camera view which I can't play from and never have set. Maybe it was a gremlin. :P

Also, if you have any other games that allow for repeat action with the left button try them out and make sure it's not the mouse.

I couldn't find any settings that may affect this behavior. Other games and Windows GUI work OK.

Any other suggestions?

no, sorry

Maybe back up your save file and uninstall / reinstall the game? That behavior is odd.

I've cleaned the entire folder AppData\Roaming\runic games and that has resolved the ptoblem, thank you.

Awesome, glad you got it working!