It appears once ADC_CONV_ERR FLAG gets set and stays set in the subsequent scanning of other channels. Reading error register does not seem to reset it as well.
The datasheet says:
"The ADC_CONV_ERR flag is set if there is an overflow or
underflow in the digital filter. The ADC conversion clamps to
all 0s or all 1s also. This flag is updated in conjunction with the
update of the data register and can be cleared only by a read of the error register."
To test this behavior when the flag is set I have read the error register twice. However the flag was still set in the second reading. So it appears reading it does not reset the flag.
The error register seems to be read only as well. No chance to reset the flag by writing 0.
My configuration is single conversion per channel. Once one channel is converted the software selects the next channel manually.
I would appreciate very much if someone could shed some light.