Shared Mailbox Attachements, form cache corrupt

Hi I have around 65 users who are getting the error message that:
"If you can see this information, your form cache is probably corrupt, Please clear the form cache."

I have followed the instruction at this link

and that has resolved the issue on the users personal mailbox but the issue still persist on shared mailboxes.

I have also removed the OST file and let it rebuild but this does not resolve the issue.

Any help would be great

Outlook 2016, Windows 7 and Windows 10.