My blog has gone since updating to 2.8.4 !

My blog: https://www.jeanlucbenazet.com/blog/

I have decided to update to the latest version today from 2.7.

After the installation and the upgrade of the database, i noticed all my plugins were deactivated, so i tried to reactivate them again, and got the following error:
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Warning: in_array(): Wrong datatype for second argument in /usr/local/psa/home/vhosts/jeanlucbenazet.com/httpdocs/blog/wp-admin/includes/plugin.php on line 310

Warning: Cannot modify header information – headers already sent by (output started at /usr/local/psa/home/vhosts/jeanlucbenazet.com/httpdocs/blog/wp-admin/includes/plugin.php:310) in /usr/local/psa/home/vhosts/jeanlucbenazet.com/httpdocs/blog/wp-includes/pluggable.php on line 850

Warning: Cannot modify header information – headers already sent by (output started at /usr/local/psa/home/vhosts/jeanlucbenazet.com/httpdocs/blog/wp-admin/includes/plugin.php:310) in /usr/local/psa/home/vhosts/jeanlucbenazet.com/httpdocs/blog/wp-includes/pluggable.php on line 850
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Worse, I tried to change the Theme, and suddenly i’m told all themes are broken, and suddenly my blog changes to a blank page!

Can anyone help? I’m not very technical… even though i managed to fix bugs in the past. This one is going over my head ??

Thanks
Jean-Luc