Thanks Lex. mikeprimm released an updated Dynmap to fix the issue.
Still showing the big red Incompatible X in the server list but I can connect. Appears Morpheus is another mod with a similar issue.
Forge 1078 is rejecting clients from connecting to the server due to missing the server-side mod Dynmap. This behaviour did not exhibit in Forge 1065.
Bugged this with both Dynmap and here.