Issue: It seems that Client PC certificate CA Root is expired.
Cause : When a Client PC connect to Server PC a Temporary CA Root Certificate is generated between client and Server PC, if Server haven't configured with RDS CALs.
Solution 1: Delete MSLicensing Folder from registry by Searching entire Registry.
Step 1. Open Command Prompt and type "Regedit.exe" and Hit Enter.
Step 2. Go to Edit and select Find…
Step 3. See below snapshot, Type “MSLicensing” and Press Find Next.
Step 4. Delete “MSLicensing” Folder and Press F3 to continue to find the same and delete all “MSLicensing” Folder from registry.
Step 5. Restart the "Remote Desktop Services" on Client PC from and then check the application access again.
NOTE: if you are still not able to access you may need to also perform below additional steps.
Press (Windows Key + X),
This will pop up a menu, select Command Prompt ( as Admin )
1.Type without quote in command prompt "regsvr32 C:\Windows\Syswow64\mstscax.dll" and Press Enter.
also register below file "regsvr32 C:\Windows\System32\mstscax.dll" and Press Enter.
2. Right click and “Run as” administrator mstsc.exe under C:\windows\System32\
3. Right click and “Run as” administrator iqclientmgr.exe and NMTSAHOST.exe (Path : C:\program files\Propalms Client.
FYI, Below is the Official Microsoft Link for all Info about MSLicensing.