TLDR - If you really need your shared folders and can't WAIT for MS to fix the issue, create new account a migrate the data is a real pain but works and will be faster and wafting for MS.
Thank for your post the info that this due to a backend migration, was very useful, some of my folders starting having the issue last year found a post that Microsoft was aware and working for fix as my most important share a 100GB folder was not affected I've just WAITED, until after a Windows and OneDrive app update wake to a deleted local folder some error about the files I've had open when the PC rebooted (forgot the PC on and Windows Update as usual don't care if it destroy work it just reboot), stared looking for a fix but at almost a year since the problem started showing up Microsoft is still working on Fix, as I'm a paying customer decided this time was worth jump to all the hops to chat with MS Support that was a waste of time even if the agent was very polite and apologetic no resolutions was reached, but thanks to your post I've suspected that this was not a problem for new accounts, so I've created a couple of free account to test this, spined up a couple of VMs and tested creating some shared folders and can confirm new account work ok no stupid open in the browser workaround, so if MS won't migrate mi account I'll do it myself, as I've a Family Plan with an unused member I've started the slow and painful process of copying all my data to the new account and create new shares (still in process I've a bit over 500 Gb in my OneDrive) but should be completed in a couple of day and not month or years, so if you really need to solve this issue this a very painful and suboptimal workaround but finally can have working shared folders again.