[Doug Tatham]

I don't have much experience with automatic transmissions, but I can think of two possibilities:
1. Most likely: fluid hadn't worked its way into the valve after change before the valve tried to operate. No lubrication: valve didn't open/shut, error code. If error presists, you need to have it checked.
2. Slight possbility: When fluid/filter was being changed you got debris downstream of the filter and clogged the valve and it's being held open.

Suggestions: With the brake pressed shift from reverse to each of the gears. Wait for the trans to grab between shifts. Repeat several times. Clear the code and drive it a bit. If code returns, take it to a trans shop.