Missing navigation month view since 2021

Since januari 2021 navigation to previous or next months is disappeared. I use the month view, but the problem also exists in other views (see https://www.tvgpd.nl/agenda/maandkalender/2021/2/-).

When I select previous events in 2020 or earlier, then I can select previous or next months (see https://www.tvgpd.nl/agenda/maandkalender/2020/11/-).

I've installed JEvents 3.6.12.

How can this be fixed?

Regards, Hans