WowAce

`Your World of Warcraft client version (11306) is too old to run this version of Decursive. You need to install an older version of Decursive`

Considering I'm using the classic version of Decursive on the current classic client, I'd say the client flavor check is not working.

At a guess it's trying to parse the toc or something from GetBuildInfo() .. there is no reason to do that when the WOW_PROJECT_ID global is available (other than making your life harder : )

X-Min-Interface custom tag is missing from the .toc so it's substituting a default of 60000 (line 963 in Dcr_DIAG.lua)

This is what prevents the current Classic alpha from working on Classic. 

It gives the popup and doesn't load at all currently

Thanks for this detailed report!

WOW_PROJECT_ID must be "new" that's probably why I am not using it^^

The latest alpha should fix that.