I have a Pentair QuadDE-100 filter and it works beautifully. I tear down and clean out my filter at the end of every swim season and my pool runs all year round.

Can you describe what you did to clean it and how you did your re-charge of the DE?

If you had a leak on the pressure side of your system, your pressure would not go up, it would go down. For your pressure to jump 10PSI like that means the filter is either picking up "stuff" (which it's supposed to do) or there's some issue with the internals of the filter.

My QuadDE uses cartridges to hold the DE unlike your FNS Plus which uses the older style DE grids. Both are susceptible to tears which can cause funny pressure problems. The top and bottom manifolds that hold the grids need to be carefully inspected for cracks and broken gaskets as they can cause a by-pass of water which would be seen as a low pressure and then, when they plug up, the pressure will rise. Also, there is an air-bleed on the top assembly that allows for air-relief (as well as the high-flow bypass air-relief valve on the top of the unit which you can do a quarter turn to open). That inner air relief can sometimes cause problems.

I'm just spit-balling lots of things that can go wrong inside a DE filter (now you know why so many people prefer the simplicity of a sand filter ).