Update mechanism tries to download update file and fails, if Squirrel SQL is not running under root account. (Squirrel was installed by root)
This would be fine, however error msg tricks user by showing "Release file couldn't be downloaded. Please check your settings.", however settings are completely fine. When you check settings, error msg after "Test connection" says: "Unable to download release.xml from the specified location". However from stacktrace it is obvious, that it was rights issue:
java.io.FileNotFoundException: /opt/SQuirreL SQL Client/update/downloads/release.xml (Permission denied)
I'd propose to download update file to home folder, and if there is new update, notify user that he should start Squirrel SQL as root (or msg about insufficient rights), if he needs to update it.