An internal error has occurred.

Thinstuff Support

In case you receive the message "An internal error has occurred" or "Ein interner Fehler ist aufgetreten" the reason can be following:

So first of all please check if the maximum number of connections have been reached resp. the installed license allows further connections.
On Windows 7 that error is basically just a notification that no more connections are allowed. 
Windows 10 is more intelligent - here you would get the message "no connection allowed ... number of connections limited"
You can also run command "netstat -a" to see if there are any established Remote Desktop connections.
We've also had some incidents where  this is caused by an certificate issue:
Check the MachineKeys directory:
Copy the keys to a different directory by taking a backup and go into the file system and also delete the files in C:\ProgramData\Microsoft\Crypto\RSA\MachineKeys\
Please reboot your machine after deleting the keys from this folder.

More information:
Tags: An internal error has occurred, Event ID 1057, internal error, interner fehler