I'm getting an error message whenever I try and open the plugin tab on the WPMU DEV dashboard

This is the error:
Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 77824 bytes) in /home/content/86/10957786/html/theroadtoretail/wp-admin/custom-header.php on line 621

I was trying to install Membership prior to this starting but I think it got interrupted mid-stream. Initially, I was getting this message for the whole site so I went to my ftp and deleted the Membership plugin. It then allowed me to access the site again.

I'd also been setting up the Snapshot plugin prior to that so not sure if that might be a factor!