Hello,

I have a client where their Message tab will sometimes disappear when opening a new email message window. They have to reboot the application to get the message tab to display again. It's pretty intermittent so it's hard to replicate at the snap of my fingers. There are no errors in Event Viewer and all add-ins are acting correctly. Has anyone seen this or knows what to look for?

Thanks. 

Hi,

Are you using a custom form when creating a new email message? If so, please try clearing the form cache and then check if this issue will happen again. To do this, please follow:

1. Click File > Options > Advanced

2. Scroll to the Developers section, click Custom Forms button.

3. Click Manage Forms button.

4. Click Clear Cache button.

In addition, we can also try resetting your Ribbon to see if it helps. To do this, click New Email button to open a new email, click File > Options > Customize Ribbon, click Reset > Reset all customizations.

Please let me know the result.

Regards,

Steve Fan
TechNet Community Support