Hi Ieuan,
I hit the same problem 2 days ago. I have a personal OneDrive account which I pay monthly, but I also have another lifetime Microsoft Office Licence under a separate account which I paid a onetime fee for. I also kept my OneDrive Files encrypted using Boxcryptor (I have since found out is now merged with Dropbox...that issue for another day!)
I noticed a second Onedrive account appearing in my file directory 6-12 months called Onedrive-xyzschool... ago but has always remained empty. This second account was created as consequence of my Office Licence I think after a Microsoft OneDrive update at some point.
Anyway, 2 days ago having worked on a file within my personal OneDrive, the OneDrive folder under Boxcryptor was removed, the changes I made to my work that day disappeared, and my personal OneDrive was now only shown under Users/myname/OneDrive. However, I now got the exact same Explorer.EXE as you above.
I checked my 'Apps & features' in Settings and searched OneDrive...Microsoft OneDrive app was there and had been updated 4 Jan 2024...the day when the troube began. I uninstalled this, then opened up office.com on Microsoft Edge browser to see me signed in under my Office Licence account (not my OneDrive personal account). I switched user to my OneDrive personal account and then downloaded to re-install OneDrive app from here.
After some installing, synching and backing up files access is now restored and the Exploer.EXE is not more....icing on the cake is that my personal OneDrive also re-appeared under Boxcryptor too - SUPERB!!
Anyway, I hope you have the same issue of two accounts and the update of 'the other one' has messed up access to your 'usual' one as was the case with me.
Good Luck.
Chris