This issue has been around for a long time (potentially UE3).
UNetDriver::Time is a float, which is updated by accumulating Delta Time during ticks. This means the longer a game runs, and the higher the server tick rate is, the more likely it is that floating point error can cause the time to never properly update.
This may cause a number of issues, but the most apparent seems to be that Handshaking fails so new clients can no longer connect.
Expected:
The client connects with no issue.
Actual:
The client connection fails, appearing as though the challenge fails.
This is only one instance of the issue, but there are potentially numerous other side effects.
**Note: As of 3825875, the above repro was worked around. The underlying issue still persists.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-53832 in the post.