http://www.ootpdevelopments.com/board/s ... ost4319511
This is by far the most common error a new GM gets when attempting to download their first in-game update. There is a file that you need in your league folder, accountsdata_gms.dat, that is usually supplied by the league commissioner upon entry into a league. If you are missing this file, you will get the "Error: could not open archive file" message when attempting to download the league update. If you misnamed this file or copied the wrong file into settings, you'll still get that message.
The accountsdata_gms.dat file contains the server information that OOTP needs in order to export your team changes and/or download the league updates. If you don't have the correct file, the game cannot connect to the server. It's also a myth that this file contains GM profile information. It simply contains the information needed to download updates.
When your commissioner sends you this file, it must go into your league's settings folder. If you don't know where this is, there's an easy way to find it. Open your league, go to File - Online League - Online League FTP Settings. Once there, click on the "Open League Settings Folder" button. This will open your league settings folder.
Now you can simply paste the accountsdata_gms.dat file into it. Ensure that it is named exactly as accountsdata_gms.dat. If it looks like accountsdata_gms (2).dat, for example, it will not work.