This usually happens if you are trying to play a PAL (Europe) game using a USA BIOS. Ensure your BIOS region matches your game's region, or use a newer BIOS version that supports multi-region better.
Unfortunately, I must advise against downloading BIOS files from third-party sources, as they may be modified, outdated, or contain malware. Instead, consider the following options: aethersx2 bios 4mb download