Photos won’t synchronise

I recently bought a new computer, which meant that I had to move my Lightroom Classic catalog from my old computer to the new one. It worked fine and all of the photos that were on my old computer are now in Lightroom on my new computer.

However, when I import a new photo into Lightroom on the new computer and try to syncronise it with Lightroom CC (on my mobile phone), this message comes up:

PROBLEM SYNCING WITH CURRENT CATALOG

This appears to be a duplicate or automatically created backup of your catalog. As it may not be current, sync has been disabled for this catalog. To make this your sync catalog, please relaunch Lightroom and enable sync.

I have of course tried to restart Lightroom Classic, but it does not make any difference. Some people say that you should remove ”sync.lrdata”, but I can not find that file using this path.

C:\Users\<user>\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata​

What can I do to solve this problem?