Thankfully it wasn't critical at that moment, but I was unable to find a solution and resorted to reinstalling the OS.
Today it happened again. It was immediately after I made files available offline and rebooted.
After a day of searching and testing I stumbled on a reddit thread with the fix.
Conditions: Windows 10 2004. Offline files synced. The network location of offline files is unavailable.
Result: A short time after login Windows bluescreens with a RDR File System, mrxsmb20.sys error.
The fix (workaround):
As per this reddit thread, there is a registry "fix" to get this working.
I think I've found a solution, on the clients with bluescreen create the registry DWORD "ProviderFlags" and set it to 1 for each offline-synced network drive under HKEY_CURRENT_USER\Network
This enables SMBv1 on client for those drives and we no longer get bluescreen on login.
Our file server is server 2016 with SMBv1 disabled, so this makes no sense at all, yet it fixes the bluescreen for all our 2004-builds...
Can you try this and see if it helps you aswell?