I’m not sure if re-signing will fix it, it’s hard to say from gpg what the error is. Most of the reasons I can think of for it to fail with an “unknown system error” would be reading the signature file (kexec.sig), which re-signing would try to replace. But it could be failing to read it due to filesystem corruption, a hardware error, etc.
You could try re-signing to see what happens, but there is a small chance this could make corruption worse, etc. Or you could check dmesg | tail -30 in the recovery shell after the error, the kernel might tell us more about what triggered it. A photo of the screen is OK if it’s readable.