I'm also experiencing the same behaviour since the update. I'm still on March 2018 version (on Desktop) and refreshing a report on BI Desktop works fine but attempting to do this on BI Server always fails claiming a #datetimezone error. Shame we can't just opt to not import the date modified and date created fields...
I've just gone in and changed the API Version clause in the source line (advanced editor) to 14 instead of 15 and that's done the trick. Saves the hassle of having to downgrading PBI as others have suggested. :)
API Version 14 brings through dates in this format: DD/MM/YYYY HH:MM:SS
API Version 15 brings through dates in this format: DD/MM/YYYY HH:MM:SS +/-HH:MM
If I'm honest I really like that Version 15 brings through the +/- HH:MM data because it would make conversions between UTC/GMT and BST much easier but that seems to break the datetime field when refreshing on BI Servers in Europe.