@tigerjack I've got to the bottom of this - it is an rclone bug in v1.75.0 
What happened: Proton is migrating Drive to a new generation of encryption and v1.75.0 started uploading new files in the new format, following guidance from Proton on how to do so. It turns out the Proton web app cannot yet decrypt one part of that format (the new-style "node key" attached to each file), so every file uploaded with v1.75.0, or a beta from after 2026-07-13, shows "cannot be decrypted" in the web app. As noted above the Android app can read these files - Proton's own clients are mid-migration and their support differs. Your data was never at risk: the files are intact and rclone reads them back perfectly; it is only the official apps that refuse.
The fix: rclone now creates files in exactly the same format as v1.74.4 did, which every Proton client reads. Fixed in this beta and this will be in v1.75.1 shortly. I have verified files uploaded with the fix open correctly in the web app. Please test it out.
v1.76.0-beta.10068.92f931519.fix-protondrive-unable-to-decrypt on branch fix-protondrive-unable-to-decrypt (uploaded in 15-30 mins)
Repairing affected files: unfortunately a file uploaded with v1.75.0 cannot be fixed in place - the problematic key is attached to the file for life, so re-uploading over the top is not enough. Once you are on the fixed version, delete the affected file from Proton Drive and copy it up again, e.g.
rclone delete proton:path/to/file.txt
rclone copy /local/path/file.txt proton:path/to/
If a whole directory was newly uploaded with v1.75.0 it is easiest to rclone purge it and copy it again. Affected files are easy to spot in the web app - they have the warning icon. Files uploaded with v1.74.4 or earlier, and files that already existed and were merely updated, are fine.