When assigning individual PINs, it is permissible to assign a PIN with more than 8 characters.
When changing the PIN, the new PIN must have the same number of characters as the “old” PIN or more characters. The BNotK card does not allow a change from a 9-digit PIN to an 8-digit PIN, for example.

In Sign Live! C.C.C up to Version 7.1.11.1, a maximum of 8 characters can be entered during the signature process with a BNotK card.

Solution:
With update from Sign Live! C.C.C on the version 7.1.11.2 is a PIN entry from up to 16 characters is possible.
Check in Sign Live! C.C.C via menu item Help> About which version you are using and, if necessary, make an update.