I'm trying to reproduce this issue, because it would be related with a multithread process which rebuilds the thumbnails in background.
In my tests, everything seems correct (no freezes), but i noticed that thumbnails are rebuilt every time Browser is opened, when they should be reused.
This is caused by a bug in the code we introduced several versions ago.
Although that is not related with the freeze issue, it would trigger the problem more frequently.
I fixed this issue. Please, try this update and tell us if there is some difference. (outdated)
For Max 2021 and before, please overwrite
ForestQtCatalog.dll in the 3DS Max root folder.
For Max 2022 and greater, copy it at
C:\ProgramData\Autodesk\ApplicationPlugins\ForestPackPro2024\Contents\plugins\2024 (replace 2024 with your Max version).