I was not able to login to Sumo Logic UI using Chrome browser, using credentials that were working previously. Looking at the developer tools in the browser (using right click in the browser window and selecting Inspect and selecting Network tab), I noticed a HTTP 500 (Server Error).
The chrome version was 74.0.3729.108 (Official Build) (64-bit)
Currently, there two known use cases that have been known to trigger the login failure.
1. When system time is changed and then user tries to login
2. Also seen as a side effect of chrome garbage collection - this could be addressed in newer Chrome versions.,
The easiest workaround for this problem is killing/restarting chrome browser window
Another workaround is to login using an incognito tab.
Finally use a different browser type like Firefox or IE or Safari