I have the same problem on Win 8.1 x64 with 3CX V. 12.5 Tapi driver:
When I dial a number from our external application, dialer.exe from microsoft starts with an error message: "There was an internal error. Close the program and start it again"
If I then don't close dialer.exe and try again, and it works.
If I close dialer.exe then dialog causes the same message again.
Is there a solution for this problem ?