TLDR:
- disable APK backup in Backup Expert Settings
- clear large apps' caches
Apologies for digging out half year post. Had/have similar issue on Pixel 7 with backing up to USB-C.
I have "Backup my apps" enabled. I noticed that Seedvault keep crashing when backing up APK for App_1, I force stopped app, then it kept crashing on APK for App_2. In both cases it was after some timeout (1 minute?). Looks like Seedvault can't get a lock (or terminate apps) on those APKs.
In Settings > System > Backup >3 dots (upper-right) > Expert Settings > Disable APK Backup
It got better, but started crashing on backing up App_3 data. That app had over 1GB of cache. After clearing cache there it finally worked to the end.
Findings:
- lock timeouts on APK are incorrectly reported as WiFi/mobile errors
- Seedvault backs up apps cache - IMO not needed
- Seedvault times out on long I/O ops and always reports as WiFi/mobile errors