Calendar Sharing Office 365

[kidpressingbuttons]

We have about 17 shared calendars on site for all of our resources (messy I know- before my time) and we have just made the move to Office 365.

I migrated the calendars and tested sharing them and made the relevant changes to the permissions. However now if I send the calendars out to all my staff and they try to open them all they get is an error message with the error code HTTP:500

Anyone experienced this? If so how did you work around it?