I tried logging on this morning and I was given the message.

"Failed to synchronize registry data from server, closing connection" 

I've reinstalled both forge and DM yet the issue persists. 

Is this on purpose or is this an actual bug?

Try logging out of your microsoft account then log back i . It might fix the issue

You need the latest update of the Dalek Mod (which will soon be uploaded to our website, it's on curseforge already). Other server side mods  (stuff that adds new blocks/mobs) in your mods folder may also prevent you from joining.