Please upgrade to the latest version Gramps 5.2.3 and check if it works?
The thumbnails should be automatically created when you use some of the addons or reports like the Narrated Website report.
When you reinstalled can you confirm you only used the Gramps XML backup with media option?
Previous discussions:
- Gramps Thumbnails 1st of July 2024
- 5.2 beta 2 thumbnails not showing August 2023