Check engine (P261B) intermittent

We've got a 2015 Prius just shy of 100K. A few weeks ago, we were just out of Houston on our way back to Austin when the check engine light came on. We pulled into Costco and filled up the tank while we figured out what to do (the tank was half full already) and when we started the car again, no light. This happened again on last Sunday; my husband decided again to fill the tank (it was about two-thirds full) and once again it turned off as soon as he filled the tank.

So, this week, although the light is off, he got an OBDII and the code is P261B, water pump. Later he found our much older OBDII, but it just says "error" (no code).

I think it was our old Gen I, or maybe the old 2003 Camry we used to have, that would sometimes throw a check engine light that would be cleared just by removing the gas cap and screwing it back on, so I assumed this was a similar issue, at least until the OBDII indicated the water pump.

We always click the gas cap a few times when we put it on, and never ever top off the tank when filling up. The last maintenance was at the dealer last October, I think the battery was replaced and brake fluid on top of the usual. Even though it's getting up there in age the car has been extremely reliable.