I had this years ago when I first installed headers without the rear O2 sensor extensions. I can't see it being upstream as your headers replace the prone-to-cracking Gen1 exhaust manifolds. Can't see both cats taking a dump at the same time either. The rear sensors, if on extensions, are unlikely to factor and when I put my extenders on it fixed the code problem.
Seafoaming the evap line I'm drawing a blank on because I haven't even so much as looked at my evap line yet. I can't see Seafoam fouling an O2 sensor though, if anything it'll clear away any deposits.
Try flashing the ECU and seeing if the code comes back. Maybe cleaning the evap line changed some input that's making the ECU have a seizure. A flash gives you a new baseline.
Brad 2001 SE V6
PERFORMANCE: Volant CAI & PowerCore, Eco fuel-line catalyst, NGK Iridiums, Kiker wires, PCV condenser, full synth, tranny cooler, AC headers, Gibson exhaust, Z31 thermostat & IMS, Jet TBS
SUSPENSION: 265/75/R16 Goodyear Silent Armor, AC 3" suspension lift, IAB, Warn hubs, King Kong steering
ARMOUR: Shrocked, sliders, tail-light guards, braided brake lines
DOO-DADS: Scangauge II, PIAA headlights