Sometimes the OMS portal seems to have various issues, so I will write about some that I encounter.
For example "HTTP Error 400. The size of the request headers is too long"
Usually this error happens if there is a corruption in the browser cache or cookies.
What to try which usually solves the issue: clear the browser cache/cookies and browsing history : http://www.thewindowsclub.com/delete-browsing-history-cookies-data-edge
-open browser in private mode (incognito)
-test across other browsers