Using 3.6.1 (upgrade planned for new version) multi user installation.
Since a couple of days, its not possible to open the library from all client computers. It's either asking for credentials (normally windows login, so no credentials required). After entering credentials it's loading library, but not opening.
Only way to stop the loading process is to kill the application by task managing.
Reinstall (rename profile to old) the client applciation didn;'t work.