I am running linux and have no problems running the game at max settings.

For the last week though, Divinity will not run if steam is running. For a few days it was OK if I ran outside of steam (by manually triggering the "runner.sh" file in the game directory) - but it suddenly started complaining about a lack of libsteam_api.so - which I plundered a copy of from another game's folder.

Not it will not even run if Steam is going in the background. I have to make sure that I quit Steam COMPLETELY before I run the game, and only then will it work....

I get a fairly detailed error in Steam when I try to launch it - here it is:

This issue is very frustrating as it has basically crippled multiplayer functionality.

No steam overlay = no way to invite my friends, or be invited

There was a similar issue reported in the Steam discussion [Linux] Game won't start; check the fix goozler describes.

For multiplayer, have you tried using Direct Connection? You can use either an IP address for that or a 'Server ID' (which the host can get in the connectivity menu after starting or loading a game, by hitting Esc to bring up the menu to click that button, or clicking on the diamond icon at the top of the mini-map).

Thanks @Raze, I'll try that.

I have managed to completely reconstruct the collection of libraries that steam bundles, and tried running steam in my native runtime using the STEAM_RUNTIME=0 parameter. This makes no difference whatsoever.

Methinks this is something screwy with the steam client.