Can see the folder in Windows Explorer but selection of the company is not permitted in C21.
It is possible that PVSW on the server died, a workstation then opened the data, and the workstations cannot “see” each other due to their network configuration. The most likely and easiest way to solve this issue is to shut down all workstations, then reboot the server(or at least restart the PVSW engine on the server). If our diagnosis is correct, that should solve the problem.
Verify that the folder name is the same as the .C21 file. If not, they will need to change the folder name. The names must be EXACTLY the same in order to be recognized as a valid 21st Century Accounting company.