Ah yes, that was because of me pointing to .gif files in the monsterfile that did no exist.
If you change the battle frame files of the green ooze from .gif to .png then the problem can be resolved without any code change

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In the coming month I might not make as much effort on this as the past month, but personally I think I got a long way already and I do intend to keep this project running for the long term.