I am seeing this constantly in the 2020 version.  I will save the current client record to the defined folder and 2 minutes later I might get either the automatic save can't find the folder or the message mentioned when the save button is pushed for the same client record.  The only way I have found to "get around" the issue, at least temporarily, is to go to File>Change Directory then click OK without changing anything.  Then at least for a while the saving function works.  It seem like ProSeries Tax is losing or changing the folder information "somewhere" and that the folder information has to be updated over and over.  This is a real irritant.  I have rebooted and have even re-installed the application upon request by support but the issue returns.  I will limp through the remainder of this tax year but I certainly hope that the 2021 version will not have these issues.