We had the exact same problem... Starting a few months ago, We couldn't connect PC2, when PC1 had QB open.  FINALLY found the solution, here's what solved it.  First, we did a manual update of QB on both desktop machines (use the help pull down, then update QB).  Then (in Windows) go to your "Start" menu, find the Quickbooks program set and run the "Quickbooks Databse Server Manager".  Locate the company files on the host computer and follow the prompts.  It worked for us!   Don't know why it changed, as we had been using 'Multi-user" without issue for several years.   But just happy its back to normal.  You can also Reference Error H202 for more detail.