[InvalidArgumentException] Level "Monolog\Level::Debug" is not defined

[ronald]

Describe the bug
Looks like backwpup breaks some logging because it works when backwpup is disabled.

To Reproduce
PHP: 8.1 | Wordpress 6.5.4 | mailgun 1.9.9 | backwpup 4.10
Steps to reproduce the behavior:
Currently I get this error while using plugin 'mailgun'. (But I think there are more situations)

1. Install & activate 'mailgun'
2. Click on 'Settings' > 'Mailgun'
3. Scroll down to the actions:
   
4. click button 'save changes' once (you don't need an API-key to reproduce)
5. open debugging tools -> network tab
6. Click on button 'test configuration'
7. See error on last request
   optionally:
8. disable backwpup plugin
9. Click on button 'test e-mail' -> no monolog error

Expected behavior
No Exception / no monolog error

Screenshots

[InvalidArgumentException]
HTTP 500 Internal Server Error
Level "Monolog\Level::Debug" is not defined, use one of: 100, 200, 250, 300, 400, 500, 550, 600, DEBUG, INFO, NOTICE, WARNING, ERROR, CRITICAL, ALERT, EMERGENCY
Psr\Log\ InvalidArgumentException
in /var/www/html/web/app/plugins/backwpup/vendor/monolog/monolog/src/Monolog/Logger.php (line 519)
in /var/www/html/web/app/plugins/backwpup/vendor/monolog/monolog/src/Monolog/Handler/AbstractHandler.php :: toMonologLevel (line 64)
in /var/www/html/web/app/plugins/backwpup/vendor/monolog/monolog/src/Monolog/Handler/AbstractHandler.php -> setLevel (line 44)
in /var/www/html/web/app/plugins/backwpup/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php -> __construct (line 56)
in /var/www/html/vendor/illuminate/log/LogManager.php -> __construct (line 189)
in /var/www/html/vendor/illuminate/log/LogManager.php -> createEmergencyLogger (line 141)
in /var/www/html/vendor/illuminate/log/LogManager.php -> get (line 124)
in /var/www/html/vendor/illuminate/log/LogManager.php -> driver (line 113)
in /var/www/html/vendor/roots/acorn/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php -> channel (line 102)
in /var/www/html/vendor/roots/acorn/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php -> handleDeprecationError (line 71)
in /var/www/html/vendor/roots/acorn/src/Roots/Acorn/Bootstrap/HandleExceptions.php -> handleError (line 49)
HandleExceptions->handleError(8192, 'strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated', '/var/www/html/web/app/plugins/mailgun/includes/admin.php', 479)
strpos(null, ''to'') in /var/www/html/web/app/plugins/mailgun/includes/admin.php (line 479)
MailgunAdmin->ajax_send_test('') in /var/www/html/web/wp/wp-includes/class-wp-hook.php (line 324)
WP_Hook->apply_filters('', array('')) in /var/www/html/web/wp/wp-includes/class-wp-hook.php (line 348)
WP_Hook->do_action(array('')) in /var/www/html/web/wp/wp-includes/plugin.php (line 517)
do_action('wp_ajax_mailgun-test') in /var/www/html/web/wp/wp-admin/admin-ajax.php (line 192)

[CodeShakingSheep]

This might be a plugin conflict. I had a similar one between Backwpup and Xcloner, see watchfulli/XCloner-Wordpress#299

Do you have any other activated backup plugins by chance?

[ronald]

All (my) plugins enabled --> error.
Only Mailgun + BackWPup --> error
Only Mailgun --> works
All (my) plugins except BackWPup enabled --> no error