Can anyone help me please. Anyone can Join my server. But me as the Admin don't let me. At the last moment this pops up.

That particular error indicates that the save file for the individual character has become overly large. Specifically when trying to load in the number of learned engrams and items in the player character inventory.

This error typically occurs when trying to transfer inside of a cluster (the timeout is much shorter for a file load between servers than an initial load.)

If you're getting that error when trying to zone into the game itself then you may need a tribe member to log on and remove all the items in your inventory, so your player file is reduced.

Also, it may be time to do a mindwipe, especially if you have S+ on your server and haven't disabled the duplicate engrams.

How does one disable duplicate engrams?

You'll have to do an ini override for each engram you wish to remove. A quick google for "Override ARK Engram" will give you some info, but generally on steam at the page discussions for whichever mod you're wanting to lcean up / control is a good place to start as prior players / admins will often have a config file you can use as a base.