Daniel Carrasco SSH v. 2.5 was designed to use of standard ssh-credentials plugin instead of storing the keys using own mechanism.
Could you check (and maybe attach screenshots) of the following:
1. Can find migrated credentials in Credentials page (JENKINS_URL/credentials/)?
There should be credentials with "ssh-plugin" added in name.
2. Each configured ssh site/host should have credentials assigned to it in Global Configure page (JENKINS_URL/configure) - is there any credential selected in the combobox?
3. Each configured ssh site/host should have "Check connection" button to verify if connection using host info and selected credentials is possible - could you check it and let me know of the results after click?
Maybe there is an error shown?
4. Are your keys using passphrase?
Of course this should be checked on SSH 2.5. What was your ssh-plugin version before upgrade? 2.4?
If the error will still occur, I think you'll need to provide your ssh config file (of course with credentials and other values obfuscated).