To the new people being affected and arriving here: Yes, this is a migration issue on Microsoft side. Accounts that are migrated to a new system cannot have locally synced folders shared by accounts not yet migrated.
I know this thread is quite long, somewhere around page 106 many of us got to that conclusion by finding differences in our OneDrive accounts. To differentiate them, this approach seems to be consistent and reliable: on OneDrive web site, go to Settings -> Options, and check the menu entries on the left:
- If the last entry is "Search" -> account is in the old system
- "Search" is not present, last option is "Restore your OneDrive" -> account is in the new system
I'll say it again: folders shared by accounts in the old system cannot be locally synced in accounts in the new system and they become URL links. When both accounts are migrated, some people claimed it works fine again. Some users even got a message saying "account being migrated" or something like that.
Communication from Microsoft is non-existent and unfortunately we don't have an ETA.
AFAIK, there is no way to enforce that migration. Creating new accounts will just overwhelm Microsoft even more. The only choice right now is to either complain and hope your accounts get migrated soon, or stop using OneDrive and switch to another provider.
Alternatively there are some workarounds using third party software, but I won't go into details because I am not using them.