When you try to log on to the RDP rdweb application from your browser, you can observe an error saying Another user of your computer is currently using this connection. This user must disconnect before you can log on.
The reason for this error is that the RDP client's ActiveX component remembers a previous login's connection which had not been explicitly Signed out from browser. This error may happen even in case you have closed the browser window yourself and are trying to log on back with a different login of the same user (such as not signing out from a session established under the older SAM login gps\kamil and logging back on with a UPN such as email@example.com of the same user).
The process wksprt.exe (RemoteApp and Desktop Connection Runtime) is holding your session. Although you may not have any visible mstsc session established actually, the process may still keep the session (note that it is not a matter of a TCP connection, because the process only maintains kind of a memory state for the browser session).
Kill the wksprt process from task manager and you can log back on.