Quickbooks was super reliable for about 10-15 years.  Now all of a sudden it is an ongoing daily problem.  I think because they have started to used the desktop as an advertising platform.  Way beyond what the software was originally intended to do.  Just a guess, maybe not but that is the biggest change I've noticed that seems to be connected.  I've had luck with restarting Quickbooks multiple times and before starting run the Quickbooks Database Server.   Some workstations link up with no problem and others require to be restarted killing all the related through the task manager before restarting and after running the database server on the main computer that hosts the database.  Maybe the above will help you.  Good Luck.