Another update after playing around a bit:
After removing the last two pages I still get the error message (this time with TypeError: Unable to get property 'nodeName' of undefined or null reference) but the 5 pages are created but relevant/required dependencies are not correct.
After adding page 6 I get my old error again: Unable to get property 'on' of undefined or null reference and the web form is not created/displayed.
After that I started all over again by deleting the survey and starting with only one page: no problem;
with two pages: (survey has to be recreated due to changes to the gdb) no problem;
with three pages (1-3): (survey has to be recreated due to changes to the gdb) -> nodeName error again. I checked on all groups and repeats but they all match. A survey of three pages is still created but the relevant/required dependencies are not correct as above.
I removed page 3 and added page 4: (survey has to be recreated due to changes to the gdb) no problem;
Added page 5: fields are added to the gdb, no problem;
added page 6: new gdb needs to be created, -> Unable to get property 'on' of undefined or null reference, no webform
removed page 6 and added page 7: new gdb, no problem.
I am now elaborating on the pages 3 and 6 but couldn't identify the problem yet.
On the side comment: Why does the signature that I define in Connect not allow me to enter a signature in the web form but when I create the form on the Website it's fine?