If I press AF button while recording video the camera hangs just after trying to focus. It doesn't matter if AF is defined for Shutter or Set buttons. Only workaround is to set AF button blank in CHDK menu, but obviously that means no AF is possible while recording video. It only happens when in 1024 quality.
Is this a known issue?
Kind of. Some older Canons have problems with AF-scan while recording a movie. As it's not enabled out-of-the-box, it's probably a not well tested feature.
My experience with AF while movie recording:
A410 - crashes in every resolution
A420, A430 - crashes in "high" resolution
Is there a solution for this problem?
Not really. Manual focus might work, but it's also less convenient.