Hello!
Why am I unable to sign into my sandbox?
This is a known issue and the sandbox team is currently working on a follow up fix for people who are still impacted by this issue. The main thread is where you can find updates as well as user reported workarounds until a permanent solution is in place:
Screenshot of the what you would see if you encountered this error:
Thank you again for reporting this! Hopefully the sandbox team will be able to use this information to release a new fix to address the issue. If you get a moment, please accept answers to increase visibility on this issue.
If you still have questions, please let us know what is needed in the comments. Thank you for helping to improve Microsoft Q&A!