The game used to run fine, but since installing a new monitor the game tabs out after a few seconds. Tabbing back in only lasts for a second or two.
Wondering if this has to do with the game being set to a different aspect ratio and resolution, as my new monitor is 16:9 and my old one was 16:10.
Not sure if this really is the cause of the problem, but I'd like to try and set the resolution somehow but can't find any config files for it. Ideas?

C:\Program Files\Steam\steamapps\common\worms reloaded\local.cfg

There's your game's resolution. You might want to try that.

Thank you, I overlooked that one. And it solved my problem! :)