In Outlook on the Web, a new Message or Meeting dialog box does not open or when it opens it is blank. This problem occurs no matter what browser or computer is used.
Please ensure that the browser is up-to-date on updates.
This problem is common when there is a complicated signature configured on the mailbox. Often there are links or other HTML code embedded in the signature that Outlook on the Web attempts to load into the new Meeting or new Message dialog box. This HTML code confuses Exchange and needs to be removed.
Once the signature is removed, a new text-only entry can be configured.
One set of instructions on how to set and change your email signature in Outlook on the Web can be found from Microsoft: