I get this error all the time as well and I don't have a backwards compatible PS2 either. Never even tried to stick a disc like that in there so I am clueless. I hope this can be resolved like the PS3iTA CFW team did back in the day when this error happened and we could still access PSN. If someone knew that dev I am sure we could find a fix as it prevents a lot of users from using reACT PSN sadly.
PS3 CECH-25001 Rebug 4.30.2 DREX 1tb iNt & 2tb eXT mM 4.40.00
Back online thanks to Primee & PSN Tool 1.0