This is a games limitation. You can only play the campaign with the mods you started it with.

Thanks to wasteland_ghost, it might be possible to salvage your save if you decide you don't want to use this mod anymore. Download his Configurable Mission Timers mod and use it to adjust the timers back to their vanilla settings, then enable both this mod as well as his. His mod should overwrite this one and let you play it as normal.
Remember to backup your saves before trying this however.

I have a question: Does the turn-timer adjustment from True Concealment also override this mod then ?

Thanks guys.

I've heard people had success with combining this mod and True Concealment, but I haven't tried it myself.

I've just tried replacing the .umad files from your mods with the ones from True Concealment then delete the extra ones and the game loads just fine.
Thanks for answering btw.

Does the error 41 appear as a result of messing with mods like this one. I loved this mod, but right after installing it my soldiers started glitching and losing action points per turn, game would not accept simply uninstalling it so I had to restart from a place near the middle of the game and lost many hours of gameplay. How ever made this mod, can you please fix it?

This mod only alters a few variables, none of which are related to your units, so it shouldn't be affecting them at all.
It's more likely that it's a bug with XCOM itself.