Hey there,
since updating to 1.5.4 I am encountering mentioned problem. Whenever I want to open bid window with some kind of argument (linking item, some chars, integers etc.) I get a message in chat box containing "MonolithDKP: Opening Bid Window for: [argument]" but window won't pop up. I've already tried to delete WTF files, caches, reinstall addon, rollback to 1.5.3/1.5.2 but even tho the problem prevails.
Anyway today I've encountered another problem. When I type down "/dkp ?" it says "MonolithDKP: List of slash commands:" but lacks another lines (ex. /dkp export etc.).
I am really thankful for any idea how to fix this problem.

After few hours and several desperate ideas I somehow ended up fixing it. The last thing I can think of is pasting MonolithDKP lua files into World of Warcraft_classic_\WTF\Account\SavedVariables so it might the fix that worked for me I guess.