User '%1' couldn't be proxied from site '%2' to site '%3'. There are no Client Access servers on the destination site that ...

User '%1' couldn't be proxied from site '%2' to site '%3'. There are no Client Access servers on the destination site that can be used as proxy targets.

The list of servers considered and the reason for not using them is:
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%4-----
An empty list indicates that there are no Client Access servers in the site configured with an internal URL.