So, I updated R using the installr package in the Rgui. Now I face two problems:
After updating I also restarted my computer, which did no help. I also checked if the newest version is actual in use as suggested in the following post Mismatch Versions.
Any idea what I am doing wrong and how to display the code again? Thanks!
Update: The problem was solved by installing the newest RStudio version directly from their website.
Meanwhile, I found the solution - the true root of the problem was in fact an old version of RStudio. Besides in RStudio it was declared as the newest version, here it was mentioned that these statements are not always correct.
Downloading the newest version directly from the RStudio website solved the problem.