When building links between Atlassian applications NTLMv2 or Kerberos is not supported and as such EasySSO on the "server" end needs to be instructed to ignore the "client" application.
If the "client" application has EasySSO as well, it needs to have similar configuration (i.e. to ignore the other one).
This can be achieved either with IP Filtering or better with User Agent Filtering (Administration/Add-ons/EasySSO/User Agent Filtering Configuration).
For that just insert the following line into the User Agent Excluded Rules :