Hi NicoN,
It is suggested that using domain admin to log on the session host with this issue, and then type gpedit.msc in the COMMAND PROMPT to check the GP report. We can check whether you have once set the limit number of connections.
computer configuration\administrative templates\windows components\remote desktop services\remote desktop session host\connections\
Limit number of connections
---If the suggestions above are helpful, please ACCEPT ANSWER. Really appreciate. This will also help others with similar issue to find this post quickly. ---