My problems are gone too, finally.
The problem was a bad (aftermarket) starter. Although it cranked the engine just fine, it apparently made so much interference or pulled so much juice that the ECUs went haywire.

Sorry for the delayed answer Source for the quote: www.volvoclub.org.uk/volvo_workshop-850techdocs.shtml -> troubleshooting with codes Considering the voltage drop: Check the pins A30 and A10 against A20, A29 and a known good ground point on the ignition and injection ecus during operating the start...

Hi, Does the ignition ECU acknowledge the engine is cranking? To perform diagnosis, turn ignition on. Ensure function selector cable is in diagnostic unit "A", socket No. 6. Diagnostic Test Mode No. 2 (Function Testing) Press test button 2 times to activate test mode No. 2. LED should flas...