Is the Main GameCube Path in GX USB1 ? Because the game folder has to be in 1st partition of USB HDD for DM to work.
Is the GC game same region as system ? Because not all out of region GC games work with GX yet.
Try GX: r1208 ,also remove your old .cfg files from GX, because they could cause issues when you update GX.
Install USB Loader GX -UNEO_Forwarder Channel 5_1_AHBPROT.wad for USB Loader GX channel, because older GX channels could causes issues. Also try different GC games to see if DM 2.6 is working.
FAQ and GameIssues
For wii games [Guide] Fat32 you do not need a wbfs partition, wii games works perfect with FAT32.
wbfs2fat.py - WBFS to FAT32 conversion