This is a known issue and seems to be happening with build 3357b. The work around for this is to install build 3143
If that solution doesn't work it may also be caused by:
refer to solution in ticket: 39493:
I just got off the phone with the customer. The customer can now save custom fonts. I believe the problem related to a Windows permission issue. The steps I followed may have just reset the permissions for the pmlcfg.ini file. This file contains path information of where the software should go to save font files. I copied the pmlcfg.ini file out of the C:\ProgramData\Amazing Designs\Apps folder and I changed the path of the font location (i.e. C:\ProgramData\Amazing Designs\Apps\Fonts to c:\my fonts) However after doing this the customer had no fonts. So I took the backed up copy of pmlcfg.ini and copied it back to the folder.
After restarting the software I was able to successfully save custom fonts
I think what happened was because I made changes to the pmlcfg file - then deleted that file and copied over the back up pmlcfg.ini the Windows permissions for that file changed.
I will make a note about this case for any future similar issues.