[TYPO3-calendar] cal 1.6.0 on T3 6.2.4 after upgrade from T3 4.5 and cal 1.5.8: "oops, an error occured"

[Markus]

Hi,

I try to upgrade our TYPO3 installation from 4.5 to 6.2.4

The list and calendar view works, but when I click on an event to see 
the details, a TYPO3 error page is displayed "Oops, an error occured!" - 
without any further details....

What is working wrong here?

Thanks,

Markus