I have seen this error when attempting to charge from an external USB connector on a small form factor PC - connected to a USB connector on the systemboard charging was successful. As such I would hazard a guess that there is insufficient power being delivered to the N900 to charge the battery and run the phone. As you are using the mains AC adapter I suggest the cable may be damaged internally. Should you have an older Nokia charger you may wish to use this with the adapter supplied in the box with the phone.