Dropbox is stuck syncing after an update, what can I do?

I posted in this thread originally (I think?) but it was moved to 'a more appropriate' thread which apparently has been marked as 'solved'.

My problem is certainly not solved.

I was experiencing a "Starting...", crash, restart, "Starting..." cycle with version 88.4.172.  I was advised to upgrade to 89.4.278 which I did.  Still the same cycle persisted.

Then I used the web settings to unlink the PC in question, uninstalled and reinstalled from scratch.  I setup a selective sync when logging in 'for the first time' and ultimately saw 'Syncing...'.  So I thought I'd give it some time and see what happens.  Well, after about 24 hours it crashed silently and restarted itself and I'm back to the dreaded 'Syncing...'.

It did, however, appear to sync a new file I copied to the DropBox directory on the affected PC.

It may be of interest that this is a Win10 x64 PC.  Another PC of mine running Win7 x64 appears to be working without any issues.

It may further be of interest that a few weeks ago I set a registry key to allow much longer filenames, i.e. Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem\LongPathsEnabled has a value of 1.

For various reasons my development projects have deeply nested directories.  There are no symlinks though, I've run the suggested checks and there are no results.