I've seen the error - not sure what it actually means, but for us it was a matter of a lost connection to the server. in our case the server was restarted while the user (me) had files open.
The reactor table lists the various add-ins (‘reactor’ was the old Conisio name for add-ins) that are being used in the Enterprise PDM vault. The add-ins will have a CLSID (class ID) value that gets cached. One scenario for the error message could be that the client had a cached value and the add-in was changed/edited etc and the cache values did not match.
Please also refer to S-029302, 'How are API (VB .Net and C++) add-ins distributed to clients in a replicated Enterprise PDM file vault?' which contains a comprehensive pdf, 'How are API add-ins distributed in a replicated PDMWorks Enterprise vault.pdf' that explains the method of managing add-ins in a Enterprise PDM environment. It describes the registry key that most likely caused the failed to read error. Basically, it probably fails to run the check to the database, maybe communication is slow or server is busy at that point.