When I first set up the BIM Server, I had user name issues shortly thereafter for unknown reasons. It got to a point where my BIM Server adminstration user name wouldn't even connect to the BIM Server despite showing up on the list.
After making two modifications, that issue has not returned (yet, knock on wood). First, we installed the latest hotfix. Have you done that? If not, try that first and make sure BOTH your BIM Server and ArchiCAD programs have the hotfix installed because different builds between the two will not connect. On a side note, if you could kindly update your profile listing the ArchiCAD versions and builds you are working from, that would answer this question in future references. Take a look at my signature for example and others who list ArchiCAD versions as well as computer information.
The other modification was disabling one of the two active network interfaces to the computer/server. Don't ask me what that meant - our IT person caught that from the
BimServer Configuration Checklist.
Again, I cannot confirm if either of these two modifications will solve your problem. But, it won't hurt to try and perhaps some of the more IT-intelligent posters here may follow up my post with their own comments? HTH.