unspecified error when changing date

When I try to change any of the blue dates I see this error message:



Soi I can't change them... I can't delete them either (if I try to delete the tags the programme closes itself / 'crashes')

Also... what are XMP-Pdf (1) and XMP-x (1) attributes? I don't think these appear in the previous version. I'm very new to this stuff. I only downloaded the older version yesterday, and this newer one today. Please forgive my ignorance...

XMP-Pdf contains the Adobe PDF namespace tags.
XMP-Pdf (1) means that there is one tag of this type in the file.
XMP-x contains a unique tag XMPToolkit

Jean