This question is not some project where I need help; rather it's a mystery I'm curious about.
At some point I discovered that if Microsoft Access project file (ADP) is installed on a server read-only share, and the users open it from there, it allows only 20 users to open the file. The 21st receives an error.
This seems to be impossible - if the users have read-only access, there's no way they would store on the share some information that would allow the next user to count how many have already opened it. Still, it's implemented, and Microsoft has confirmed it, however did not say how (" Our method for determining the number of users in the file is not exposed for customer/developer use, and there is no plan to make it publicly available."). I already posted about it here on e-e, see my comment at https://www.experts-exchange.com/questions/22091003/Access-Project-file-adp-with-SQL-20-user-limit.html
for more details
Does anybody have a clue how this might