Hi Guido
You may try this
Change default template for other one than Onepage it could be casiopea or any joomla default, then go to:
Extensions> Extension manager> Manage> Tick Onepage template (to be deleted)> Click "Uninstall".
Ensure that the template to be un-installed is not a default.
Then install the new version of Onepage
As for the /com_sppagebuilder/parser/addon-parser.php') errors
I solve it by going back to SPP v3.8.10
In extensions disable all SP Pagebuilder items
Then renaming the com_sppagebuilder folder to com_sppagebuilder-v5-8
And then install the SSP 3.8.10
I know its not the best but i guess you are in a hurry