Adding node to 800xA system pops up message "Class not registered"
"Class not registered" message appears while connecting a node to the system.
Most likely, some required software component is missing, or is damaged.
Start off by performing a DCT Software Compare of your system
Diagnostic Collection Tool -> Analyze -> Analyze Software -> All Nodes In System -> Compare with known good node -> this node missing something?
Often, exporting the Software Compare to Excel eases viewing/filtering the result.
-> Install missing components.
If no missing software is found, or installing a missing component does not help you probably have to start reinstalling software.
You do not mention version of xA; if you have less than 5.1 you must manually identify & install 3rd party software according to 3BUA000500. At 5.1 and later, the "Common Third Party Software" MSI kit contains all external requirements. You may try to reinstall this MSI kit once again.
You may also have to remove and reinstall 800xA Base + service packs + rollups.
Contact your regional ABB support center for assistance.