You might've gotten a crack in the radiator header, or else there's a blockage there. If it's a leak you're done, buy a new one. If it's a blockage, you might be able to jar it loose by taking it out of your case and tapping it with something lightly or rapping it on the desk. It's a good cooler but if it gets clogged it's an all or nothing deal and you can't take it apart to fix it.

Thanks for your reply! I'll disassemble it after work and see what might be causing this.

Ok... The problem was my table. For some reason, when I put it under the table (huge amount of open space, coolers are not blocked from any angle), hot air stays under it and cool air doesn't push it away.
It's sad though, since I built this table with my PC in mind, and it failed tremendously. I expected that it will run hotter but not 15-20 degrees hotter, that's why I thought my h110 is failing me. There is 0 air flow under my table. 😕

You could try an alternate solution if you want to leave the case under the table. Just a thought, I did this twice and it did work well both times, once for an entertainment center and once for a computer beneath a desk just like you. Cut one or more holes in the table where it makes sense and install one or more 120V case fans wired inline with a slider for volume control. Plug them into your wall outlet. Worked nicely for me but certainly not as efficient as getting the computer case in an appropriate ambient condition. However it does move hot air out from under the table if installed appropriately. I suppose you could instead install 12V PWM case fans in the desk and control them from your computer but they won't move as much air as line voltage, and you're displacing a lot of air when you have to cool the underside of a desk.

But yeah, you do need airflow. 🙂 I am still using that same desk but have disabled the 120V fan because I now have the computer sitting on top next to the desk.

davemon covers it all pretty well.

But another possibility:

The AIO cooler could be performing very much the same as it always has. But the CPU could now be putting out a lot more heat than it did before at same performance load. An overclocked CPU burns bright and fast ... and also burns out over time.

Failure in motherboard VRM hardware isn't uncommon and often passes heat to the processor. If the VRMs are running hot then you may need to replace a bad cap or repaste the heatsinks.

And of course the CPU may need repasting. TIMs generally cook off after a few years (at most). I'd recommend Arctic Silver 5, it's not the "absolute very best of the best" TIM but it's still an amazingly consistent top performer, it has perfect properties for your particular CPU-AIO surface interface, it's cheap and readily available, and (with proper application) it tends to keep working efficiently for years at a time.

You of course know that freshly-applied TIMs won't perform at their maximal efficiencies until a period of "cure time" has passed. For AS5 the full cure time is about a week, although it's still not too bad immediately after application and rapidly gets better over the first few hours.

Your Corsair Hydro H110 cooler comes with a 5-year warranty.