That is probably what the wp_mkdir_p() function was not working and it was not possible to activate the plugin, because there was already an exceeded disk quota, that explains it. The problem was very likely present before the plugin activation.
The error log is disabled in your wordpress installation, and it would report any errors when a plugin is activated. I don't see any PHP error logs, or any other big files in the wordpress directly, everything looks okay there. I also checked the upload directory, that looks clean as well. I've looked any directory available, but nothing. Maybe you should try looking for big files via SSH. This issue might not be wordpress related, as I don't see any suspicious big or high number of files yet.
No wp_mkdir_p() is different from that. That error means that WordPress is not able to check if a directory exists or not, because there is a restriction. The error is coming from the WordPress core. It very likely means that WordPress is not able to access the directories within it's installation, I'm not sure.Best,
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