rtags.el doesn't produce any diagnostics anymore (flycheck in turn neither)

[root42]

Since a while I don't get any diagnostics anymore from rtags.el. The RTags Diagnostics buffer is always empty (even when there are errors) and flycheck can't display any errors either. This might be related to #1428, as I have to disable the rdm diagnostics output, or else rdm will crash.

Steps to reproduce the behavior:

1. install current rtags via brew
2. enable rtags.el in emacs
3. visit any project indexed by rtags, which contains errors or warnings

Expected behavior
I would expect to see diags in the RTags Diagnostics buffer and highlights by flycheck.

• OS/Version: Ubuntu 20.04.4 LTS
• RTags(rdm,rc): 2.38.130
• Emacs: GNU Emacs 27.1
• LLVM/Clang: 13.0.1

[root42]

Update: I recompiled rtags with brew using llvm@9 as a dependency instead of llvm@13 and now I have flycheck error messages again. There seem to have been some breaking changes in the last llvm release(s).