Strict standards message - even after deinstall plugin

Hello, I just bought the plugin and got a strict standard message. The message pop's up, even after deactivation of Appointments+. When I installed it again, it even popped up twice.

I already checked the debug modus, this is set to false. My webhoster cannot change the php.ini file. I also got the advice to add "php_value error_reporting 30179" to the htaccess but that didn't help either. I noticed that the message is also shown in the htaccess code on top. If I delete it, it comes back after refreshing the page.

My first question is: can I get rid of the message that pop's up after deactivation. After that, I would like to know how to get rid of the message while the plugin is activated. I hope you can help me out.

Kind regards, Coen de Goeij (The Netherlands)

This is the message:
Strict Standards: Redefining already defined constructor for class AppointmentsGcal in /home/deb75859/domains/schoolofmusic.nl/public_html/wp-content/plugins/appointments/includes/class.gcal.php on line 26