File Explorer not accessing files

Hi.  I have had Dropbox basic for many years running trouble-free on 3 devices (phone, laptop & pc).  On the pc and laptop I access files in File Explorer, where all my Dropbox files are in one set of folders.  I have plenty of space on Dropbox.  About a week ago I found that files updated on the pc were not syncing with the laptop or phone.  Files updated on either the laptop or phone were syncing with each other, but not with the pc.  Looking into this further I found that the files are syncing to the app on the pc, but just not getting through to File Explorer.  I have signed out and back in on each device... no change.  I have signed out and tried to re-launch the app on the pc, but I get this message: "You're trying to add more devices than your plan supports.  Upgrade to Dropbox Plus to stay signed into more than 3 devices at once.  Or sign out of some devices."  However I'm obviously only signed into 2 devices, so this doesn't make sense.  Any idea how to resolve this?  Thanks.