Hello Michael,
to give you a quick feedback on how things are going:
We are currently updating to the latest SP of SAP_UI. It seems that we need to have at least SP12 to be able to use SAPUI5 version 1.28.*. Currently we have SP8 installed, so that might be the overall issue.
Thanks a lot for your help, I really appreciate it.
I'll let you know if I could solve the issue when the updates are done.
Regards
Tobias