Hello there! I have feedback and some good news. Like i said, i replayed university with traveler and all the other mods and found out the error occurred somewhere between receiving the report card and the graduation ceremony. Fortunately i could get a part time job at the university so i tested some things like resetting the town, not drinking any elixirs, cancel receiving the report card and not inviting any sims for the ceremony. I believe that's all i tried and wasn't sucessfull. Except for dropping out university before reciving the report card. When coming back to homeworld, my sim still has its degree and can apply for any partime jobs.
Eventhough the problem is fixed (i might have to adapt in doing this everytime i get a new degree - but i don't mind much) i am still curious to what it might be and if i can somehow prevent this. Of course, it's not a priority so the topic can be considered "fixed" if you wish to do so.

Unfortunately I am not able to recreate this. Sim gets the degree then the job without error. If I had to guess something wonky happened on that travel transition back. If you re-encounter the issue please provide me with the save file so I can take a look.

When you mention something happened in the travel back, which travel are you refering to? If it is Neighborhood -> University , i did travel with the traveller mod on so i'm not sure if it should have prevented the error coming up later in some way.

Since it was my first time trying out the sciense skill i did this in my previous town (could possibly be causing this too? Since Sims 3 is known for giving out errors when the cause of it can be be so random i like to always mention it, just in case...):
I had a DNA sample of the tragic clown and i figured i could create a clone of him once i got to the next town, so I saved this sample in a chest from world adventurers when moving towns. And once I installed them on the new home in riverview i no longer had the sample and it got replaced by a random rock or plant one. Maybe my family is carrying some corrupted character file around without me even knowing? I don't know. No errors popped up about any tragic clown until now though.

And that's alright, really. Thank you for giving it a try, anyways! And i shall provide you my save if it happens again.

I was referring to the trip home since replaying while at university and back home seemed to resolve the issue. I don't believe the science skill is related but you're right about this game's issues often not making any sense so anything is possible. As for the clone, that system is pretty messy and I'm not surprised it vanished. I don't think any of those survive travel.

Well, i mean it got resolved 'cause i backed out before receiving the report card. But, yeah it works i guess. I just won't have a shiny diploma on my wall but that's okay
Apparently they don't, yeah. Sim samples seem to be the only exception at least from what i've seen. Rocks, plants and fish do seem to work. I'll test some things for myself a bit more if not then i'll just play until the next time my sim takes a degree and if the error happens again i will come back here with the file.