Syncing Freezes in LRC on iMac

I have just returned from a trip to Asia. At some point I accidently reset my camera to shoot Raw and JPG, rather than just Raw. I deleted all the JPGs in LRC, but they then reappeared.

I have now tried deleting them in the LR desktop app. The result is that LRC continually shows that it is syncing, but the number of photos being synced doesn't reduce and it freezes - I have to pause syncing and then unpause the get the little blue circle to rotate again.

This has been going on for days and it is a two-way problem. Changed made to LR on the web don't sync to LRC and vice versa. However, changes made in LR on the web do sync to my mobile devices and to LR (not LRC) that I run on my MacBook Pro.

Any thoughts, please?