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-2501A [CFW] & CECH-3001 [OFW]/ PS2 SCPH-3901 & SCPH-7501/ PS1 SCPH-5501
Mod Cobra 4.46 v7.00 w/ E3 Flasher [CFW] & Cobra ODE 4.50 [OFW]/ Mod HDD-OSD [Phat] & Modbo 4.0 [Slim]/ Mod MM3
HDD 1tb [CFW] & 640gb [OFW]/ HDD 640gb & OPL v0.92 w/ 1.5tb SMB