@aquapuls
Its quite likely that the AutoBoot ROM is garbled and requires either re-programming or replacing.
You can find the BootROM on the install disk of the adaptor. you can use this a bin image and program the another 27c256 (it maybe a 27c128) just an educated guess.
Sadly I cannot log-onto amiga.resource.cx to confirm that .
I can confirm autoboot+defective ROM gets you to black screen on this card. I have MC302 and MC702, and autoboot+wrong program in ROM makes it a black screen.