I had an Allen-Bradley CompactLogix (Legacy) connection with status Connected: Protocol: EIP - Run Mode that went unconnected over the holiday weekend. Instead of repairing itself it just stayed unconnected until I edited the concurrent requests from 2 to 3. Subsequently I changed it back to 2 and everything is fine now.
The number of requests that Ignition will try to send to the device at the same time. Increasing this number can sometimes help with your request throughput, however increasing this too much can overwhelm the device and hurt communications with the device.
But what does that actually mean in the context of Ignition? Why would Ignition try to send multiple requests in the first place? Why would changing this value from 2 to 3 fix my connection issue?
The change in value probably didn't do anything. An edit -> save with no changes would probably have done the same. Likely something in the driver was 'stuck' somehow, and the edit -> save caused a shutdown -> startup sequence to happen on the connection that fixed things.
No idea if that's a fixed bug in a later version or not. But I'd be pretty confident you just "turned it off and on again".