Calender doesn't allow to change month

Our Joomla is 3.8.12
Since I had to add some events I checked JEM version it was 2.2.0
I dated it up to 2.2.3.
Everything seemed fine. But the displayed calender doesn't work properly.
There is no more possibiltiy to increment or decrement the month.
On to it shows "Aug Sept 2018 Okt", but when I click on every possible position in this line eg. Aug or Okt nothing happens.
you can look at it:
zugzwang-muenchen.de and go in the menue to "Termine"

With the update to JEM 2.2.3, there was a change in the calendar modules. There were two different modules before: a normal and an ajax module. Which one did you use?
In backend there is a notice if you go to modules and search the calendar modules.

Thanks a lot jojo12.

I had the (non ajax) Kalender active.
No I tried to activate both calenders and both show up.
But both callenders don't allow changing month or year, problem stil exists

Then there must be an interferwnce between a foreign plugin or template. Please control with the official ones on a clean joomla.In my jem 2.2.3 on joomla 3.8.12 all works best.

Hi Jojo12,
can you please give a hint what part of .css is important, or give some more explanation why the 'buttons'? are not working.

normally it is not css. Mostly there's something with jquery.
which template do you use?