500 error when logging into wpadmin and 403 WPMUDEV Dashboard Error

Hi - we've been experiencing a strange problem where users can't log into their dashboard. This is affecting all sites. We can't log in to turn on support access. We see the following error in the wordpress logs

[WPMUDEV Dashboard Error] HTTP Error: 403 "Forbidden"

Can you provide any guidance? Are we blocked - or perhap is our IP blocked? I can provide it if need be.