ARDUINO: Problem with geting custom atsame51j18a board to work with ARDUINO IDE

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Hi,
I've made a custom board based on the atsame51j18a microcontroller. This micro is already in use by Adafruit in their Feather M4 CAN. My problem is that the code in arduino seems to not run at all. I mean the custom build of the UF2 bootloader runs as it is supposed to and some bare metal code (simple blink) also runs without any issues. But when I flash the code in arduino (which flashes without a problem) it doesn't run. Can anyone help me? The arduino board files are attached. (variant.c is actually variant.cpp but I cannot attach .cpp files)
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