Hi Jan,
In 2019 R3, when I added any new .rptdesign
files into the corresponding directory, the report option will automatically
appear in the Report menu:

However,
when I tried the same thing in 2020R1, there is no “additional” menu appear; And
when I tried to generate report via “Generate Custom Report…”, the following
error message was prompted:

Any idea what might I missed?
FYI, I have successfully installed BIRT add-on in 2020R1 and was able to open .rptdesign files in the Report Design perspective.
Thanks!
Chi-Seng