Accept button in email to accept Shared Calendar greyed out

[Steven_Parein]

We recently migrated from an Exchange 2010 at a supplier to O365

We migrated by using IMAP and PST (Calendar and Contacts only)

When I share my calendar with my boss he receives an email but the Accept button is greyed out.

Our supplier did not yet encounter this problem.

All of our users have this problem.

If they add the calendar manually after receiving this mail it works without any problems.

But we want them to accept through the invitation mail.

Anyone any idea how to solve this.

I attached a picture.

[#2 · Accepted solution]

Contacted Microsoft Support and there is a bug when using a disclaimer which adds itself in the mail when sharing a calendar.
We deactivated the internal disclaimer for the moment and it works as intended
We are waiting for Microsoft to unbug this bug