Microsoft Online Services: Cannot submit forms on Admin pages

Problem:
When trying to work with Microsoft Online Services Administration pages at https://portal.microsoftonline.com/Admin, after submitting forms that contain text fields, you might get error:

Sorry! We can't process your request right now.
The service is temporarily offline for scheduled maintenance or we're investigating a problem. We're working to get it back online as quickly as possible.
Error code: 0.

Such form is, e.g., Service Request.

Solution:
After quite lengthy problem solving session with Microsoft support and bunch of test service requests it turns out that if you have string like <XYZ> in the text fields, submitting the form will not work.