Accessing server as "anonymous" and with name and password

I installed the server 0.9.41 beta as an "anonymous" server a while ago, tested it with the http://ftptest.net/ site, and there were no problems. Several people from various locations were able to access the server using browsers (in one case Firefox, in the other I have to assume IE).

Now I have removed the anonymous login and replaced it with two passworded users. Both show no problems (including access in passive mode, i.e. with a browser) after testing with the http://ftptest.net/ site.

But for reasons unknown to me (server listens on Port 21, passive range of ports 5000-5100, both entered in the router NAT), two of my passworded users either cannot access the ftp site at all, or if they are able to enter it, are thrown out if they click on a file in the root directory specified in the server configuration. The user who could log in reports that a login using Chrome as a browser was impossible, and the problem mentioned above (thrown out) was experienced when he used IE as a browser. The other user, again presumably using IE as a browser, claims he could not log in at all.

So as long as the site allows an "anonymous" user without a password, it appears to work perfectly. But if I remove the anonymous login and replace it with two passworded users, they have problems.

Any suggestions as to where to look?