I use IdeaMaker as my slicer. I am running Blender on Ubuntu 20.04 Desktop. Upon completing a model, I save it and export it as a .STL file. Until 2.91 I had the same results if I exported using the 3D-Print tool->Export or the File->export->STL. Now I get an “Invalid Model” error from the 3D-Print->Export file. If I copy the model to an empty session and do File->Export->STL process IdeaMaker loads the file just fine.
I am not able to upload example files, but have the results of .STLs from both processes