The error returned when the thumbnail is pink, (which indicates it's corrupt somehow) is the same one you're getting when trying to export. If you weren't at 10.6.1 and publishing from 2.5 at some point, it may have become corrupt another way. The best the software can do is let the export go through and tell you what thumbnail is corrupt.
I looked through that case, and I think there was an issue with Server's exportSite process that used the wrong TEMP directory. It should use the path defined by the TEMP environment variable. This is present in 10.7.1 and fixed at 10.8.
The thumbnail in the arcgisinput folder would be corrupt, but the process to create the thumbnail is the same between My Content and Manager. I'd suggest using ProcMon while exporting to see what the last service was that was exported correctly. The next one listed alphabetically could be the one with the corrupt thumbnail. You can use the Admin API to update it or delete it:
Edit Item Information—ArcGIS REST API: Administer your server | ArcGIS for Developers
Out of curiosity, are you only exporting Server, or are you creating backups of Portal and Data Store as well?