No problem at all - I very much appreciate your time and friendly advice!
So I tried the update function that you suggested. I also re-installed R and
RStudio.
I changed lib.paths and checked whether old paths exist (which they don't -
also it was a brand new installation, so it would be very odd if old stuff
existed...)
I installed the tidyverse via the AUR now, which seems to work, but not solve
the underlying problem (and also does not fix the same issue when trying to
install other packages...)
At some point I ran into a gfortran problem (gfortran being missing), which I
cannot reproduce right now.
I tried installing the packages in vanilla R (so not RStudio) and ran into the
same problem and same error message - so it's not the RStudio IDE.
I think that's all I tried so far.
Also the knitr persists on both systems as well, which I didn't even know
where to start trying to fix it, since the error message does not make sense
to me.
If there is anything else I can do to help solve the problem, please let me
know! I very much appreciate your time and help!