I've seen this too and will be fixed in the next release.
If I leave the template builder open and unused for a while or press back from the main landing screen, trying to work on it consequently sometimes brings up this error message or a similar one.
Any one of the below seems to resolve this issue :
1. Update the URL to https://dpx-uat.motorolasolutions.com instead of the cached https://dpx-uat.motorolasolutions.com/Account/Login?ReturnUrl=%2f and launch the link again.
2. Clearing the browsing data [ex, CTRL+SFT+DEL on Chrome] resolves this issue.
3. Sometimes a simple F5 also seems to work in Chrome