I presume the reason for this behaviour is the fact that Start/End Time in Outlook/Exchange is saved as a serial number. Is there any solution?
Are you creating an event with the Workflow? Is this in a SharePoint Calendar or are you using this with Outlook integration?
The end time is a date-time field. Are you supplying both values?
I found the solution after a hint from a collegue: It seems to be a SharePoint bug (which was described firstly in 2008 but in a programing context! See Link : https://social.msdn.microsoft.com/Forums/office/en-US/31e84d74-3ea8-44df-86dc-2dc62381ab3b/set-endda...
If you want to change the "End Time" field you have to update additionally the field "start Time" (and vice versa). Now SharePoint accepts the modification!
Well that's two hours I'll never get back. LOL. Thanks for the suggestion Stefan Lischker Worked for me.