lookup multi select not working

lookup multi select not working

I'm linking 2 forms: 'musicians' form needs a field called 'Availability' which looksup dates from a 'calendar-type' form and allows selection of multiple dates.

This works fine for singleselect, but is not working with checkboxes or multiple list:
- on Add, it appears to submit successfully, but the data doesn't load in view
- on Edit, displays message 'invalid entries found, please correct and submit again', and also 'null Failed to update data.'

Help! Thanks.