I opened the area were the control board is located and found that a small piece of wire had landed on the RAM memory. I removed the wire and tried again and the problem persists.
It looks like the PCB is conformally coated so it's unlikely a small piece of wire could do any electrical damage. But once it had landed, it's obviously not present anymore in the place where it came from, making the circuit there broken. Perhaps that makes the CPU program confused.
It seems there are at least two PCBs inside the oven, the "power board" (504078.xx, this is what your image shows) and the keypad interface, so called "control board" (504077.xx, something like on the attached image). There are some (logic?) ICs on that board as well. Perhaps it's a keypad-related problem.
The other possibility is that the firmware in the 27C512 part is corrupted (or the MCU is unable to read it). You'd already advised to carefully check the power supply voltage for that part. If the voltage is lower than required, it can be problematic for the MCU to fetch the instructions/data properly, it's a known feature of the UV EPROM chips.
Since you'd managed to read the firmware from the 27C512 to a file, you can convert it back to the symbolic format (fortunately, it's an old architecture so there must be a lot of available disassemblers, and the code size is relatively small) to make sure the content looks sane. Generally, a FW image contains the code area, the constants, and the empty areas (that usually reads 0xFF). So you can analyze the code to make sure it's seemingly correct because there are no obviously erratic jump instructions, for instance. It can take time, of course, so it's a last resort method. But it's very powerful, you can identify the corrupted areas and fix the code manually. Moreover, you can design your own code.
Not sure about the DS series, but it seems the later series has a dedicated connector for diagnostic/configuration SW tool like Miwe BPM (baking program manager). Anyway, there must be at least debug UART pads somewhere on the board.
The wiring diagram for a Miwe DS series oven. Not sure if it's about the exact model but definitely of that era:
https://xn--b1agge9ai.xn--p1ai/one_pdf/miwe?pdf_id=5121&producer_id=188One more link. Not about your oven at all, but the DS series are mentioned. Just FYI.
https://tehnokitchen.ru/assets/vendor_pdf/resources/384883/3e9547f4a76df6fd98cd57b0d7094ebb7b6f2b53.pdf