Thank you, I'll take a look! I was also looking for an original manual if possible.

No. Just one behind the reels.

Have you looked at the Bally EM manual in the NLG file system?  Also found here: https://rwatts.cdyn.com/download/bally_em_manual/bally%20em%20manual.pdf  It includes info on the 808 double up feature unit specifically.  A lot of the mechanical diagrams show common components and the generic schematics pretty much apply across the lineup.  Although a little confusing, as it shows all the feature variations you don't have, it still includes a lot of good starting points for things to check.  Specific questions are usually helped by posting in the Bally EM forum. 

You might try taking your mis-behaving board and try it in another s+ and see if the problem follows the board.

The clear chip would be my goto.
I might also suspect that one of the pins in either the game or reel is bent under or simply not in the socket properly.

Since you have several S+ s I would also swap in a different set of game/reel chips and see if it works as it is possible that you have a bad chip.

If it will boot with the diag, go to the deployment utility and reinstall the OS.