In the Husqvarna LCT, the threads on that tube are straight rather than tapered pipe threads. The tightening is the friction at the end of the tube where it lands on the inside flat face of the end of the cap. It's compounded by friction on the threads especially when it's overtightened.
What to do:
-- You can safely remove the tube from the engine without risk. Be prepared for the oil falling out, maybe by tilting the machine a bit and adding a sheet of plastic something to direct the oil to your catch container.
-- Then the tube goes into a vise, where you can get a grip along the length rather than trying with just the small area where your vise-grip pliers are clamped. Use a socket on the cap, as that distributes the force more evenly around the cap. For sure using and open-end wrench is the least-best option.
-- You can tip the tube end-up and add some of your favorite penetrating oil inside. A 50/50 blend of ATF and acetone is a perfect option for this. Let it sit for a day before you get after the nut too hard.
-- You can add heat to the cap itself with a propane torch, while trying to keep the tube cool. Heat the cap fast if you can, so the temp differential expands the cap off the tube. Maybe a wet rag around the tube to keep it cooler. If you filled it with the ATF and acetone, get all that out before you grab the torch.
When you reinstall, there's a case for adding a bit of loctite to the thread going back in the engine, to help a little with sealing and have less risk of the tube coming out next time you change. Clean the threads in the block and on the tube first. The loctite wants a day to set up undisturbed if you can. On the cap end, add a smudge of teflon thread sealing paste on the end and the threads of the tube. I use the same stuff you might use for a tapered pipe connection. Tighten that cap "just enough" rather than "that's not going ANYWHERE!" tight. The paste in the end joint and the threads will make things a lot nicer the next times. No leaks, no sweat.