Invalid constant, the sim just freezes before playing violin and the error message pops out.
I... have no idea which mod could be the reason for that. I suspect the "pack of custom violins" but I'm not sure - because this one I'm using in the save is the Maxis original one, just with the recolor that I've already posted two years ago.

I am now afraid that some of the mods broke my save file or something (but probably overdramatising here)... Or at least banished me from playing violin. Which is still bad. Double checked if I had argon's custom instrument mod to be sure and yes I do have it in my game, even before the error popped out. It's strange, because it never did this before and my sim played the violin just fine, so I want to make sure if it's really because of the custom violins (but technically how tf do they relate to this? I wasn't using them on the lot)

Let's just get the obvious out of the way - have you tried removing the custom instruments to see if those are the cause?

The error also mentions formal wear and being in the zone. A trait mod perhaps?

First step is to use 'What caused This" as you did not mention using it. If you don't have that program then you need to get it.
https://www.picknmixmods.com/Sims2/...CausedThis.html

You have some hack that is overriding constants defined by Argon's hack. You should be able to find it with HCDU+.

After tinkering a bit, thank you all for your advices - Turned out, that I'm dumb enough, and downloaded Atavera's instrument hack, at the same time with Argon's hack. EVEN THO I KNEW THAT BOTH OF THEM SHOULD NOT BE TO USED AT THE SAME TIME.

I am an idiot, but I'm glad that there is people who can and want to help dumbasses like me.
I also deleted the custom violins while testing, because, well, whatever, not a big deal, will redownload later. Most importantly that the error is gone, all thanks to you and your advices. I actually forgot about the WhatCausedThis and HCDU+, next time will make sure to use it first before jumping on forum. So case is closed!

Happy noises