Thank you for your help and please excuse my late response, I‘ve just returned from my summer vacation. Concerning your questions:
The issue occurs in all pdf files.
The only thing I can do is to add annotations (without highlighting any sections of the text before). Everything else like underlining, highlighting etc. does not work. I do not get any error messages though.
The version I use is 19.07.01 (20190730.171831) on IOS 12.2.
I hope this answers your questions. I highly appreciate your help!