Please find more information here.
I went to join a meeting and when I began to type the meeting id there is an "m" inserted as the first character. The meeting id had no "m" in it.....and I don't know why this is happening every time I open TeamViewer
the TeamViewer meeting ID has always an "m" before the meeting ID numbers (to help to distinguish to a RC session). Sometimes people only copy the numbers, and to prevent confusion for the meeting participant, TV is adding automatically the "m" when typing in a number in the meeting window.
I hope this helps.