Scheduled scripts failing

Hi,

We recently installed PSU, and when testing scheduled scripts they are all failing immediately with the following error:

Cannot convert '' to the type 'System.Collections.Generic.ICollection`1[System.String]' required by parameter 'Collection'. Specified method is not supported.

This happens with all scripts tested so far, including all execution environments (PS 5.1, 7, and integrated).

Any help would be much appreciated.

Thanks