Hello,
I had a same problem in the website of my company.
Il I upgrade to v5.5, il can’t use the admin interface.
I had to desactivate manually the plugin and revert to v5.4.2.
On the plugins page, I had a message when I try to reactivate de plugin :
Parse error: syntax error, unexpected 'print' (T_PRINT), expecting identifier (T_STRING) in /home/hrefgzuu/www/wp-content/plugins/contact-form-7-old/admin/includes/welcome-panel.php on line 9