After the problem with 2.27 and windows slaves I tried 2.28 but ran into another new problem.
Executing a windows batch file, the path separators are not converted to backslashes:
Log from 2.26:
set FOO=T:\Jenkins\workspace\...
2.28:
set FOO=T:/Jenkins/workspace/

This does work for setting env vars but not when I try to call a windows executable/shell function:
if exist T:/Jenkins/workspace/myproject/foo rmdir /s /q T:/Jenkins/workspace/myproject/foo
--> Invalid switch - "Jenkins". because '/' is interpreted as a command line option