UpdateChecker.php warning

Hello,

The plugin was working amazingly and I wanted to add some more feature as an addon.
Recently I purcashed the Login and Datepicker add-on.
After I installed and activated these addons, I get the following warning at the top of the wp admin panel:

Warning: strpos(): Empty needle in /home/jkiatz/public_html/workabroad.fullonweb.hu/wp-content/plugins/participants-database/vendor/aux-plugin-update/Puc/v4p1/Plugin/UpdateChecker.php on line 454

It appeares after I activate any of the extra addons. It disappeares after I disable both of them.
I clicked to the empty page cache button after I activated them.