When I try to open by UI Action report, I get an unhandeled exception error. It is 2020.1.1339 stable version of web client running on Azure portal.
When I try to open the same report via UI action button on local client distribution, it opens just fine.
Seems to me that maybe the data types are not correct? Have you attached an XSD as the report source? It helps to set the right data types to the data fields.
Also version 2020.2 has an updated FastReports engine. Maybe it is worth the try. It requires a model update though, so it is not backwards compatible.
I did in FastReport designer and it seems to me all data types are correct. I thnik strange thing is that it only does not work on Azure On other server with different versions of Origam it runs just fine.
Also you seem to be running the old server backend (.net 4). The current HTML version backend runs on .net 5 (version 2020.2 was based on .net core 3).