Hello m_flo and welcome to AF. That's a new one on me. Are you using the stock htc calendar? If so, I'd consider disabling it and trying something different from the play store.
They are all written to use your google calendar and many are much nicer than stock.
As for a cure to what you have running... wow. I wish I had something clever to offer. You might clear the cache or data and then set it up again and see if it behaves.
I hope someone that uses the calendar more has some better suggestions. Best of luck to you and thanks for using these forums.