Same issue... It seems that it will eventually redeem the code if I keep trying, but I've lost track of how many times I've had to do it on some of them (at least 7 times). When it does finally work, it seems I get an error saying the code was already used, but at least it did finally activated it on my account by the time I get that message.

It's not just code redemptions I'm noticing this on, it seems GOG on a whole has been running extremely slow (even certain games I already own aren't showing as I own them when I look at the store pages for those games), sometimes pages won't even load correctly (showing some character holding the world over it's head; this happens in other areas other than the store, like profile, community & chat pages).

EDIT: With the exception of the Quake II redemption; it's reading that the key was already used, but it still isn't showing as I own it when I look at the store page... I guess I at least own it on Steam if I have to go that route, provided it continues to run on my OS (this particular reason is why I stopped purchasing from Steam)...
UPDATE: It does appear to have been added to my account when I look at my order history however.

The fun part is picking out bridges, bikes, and stairs two times for each fail.

Seems to be the only indication that the redemption might actually work... Ironic the last redemption (Quake II), I had to go through 5 or 6 (on this 1 game, not including 2 or 3...or 4 prior when it didn't work) before it finally attempted to redeem the code successfully (with another error of course, but seem it did actually activate after doing some digging)...

Oh yeah. So over that!!

If you use the GOG Galaxy app to redeem codes, you don't have to go through that.

It does, however, fail to work just like the website.

Same problem here. Tried to activate through Galaxy. Galaxy stops working and the activation just stuck in a load loop. Have to kill the program. Now I have a "code already used" msg and the game doesn't show in my library.

Same here, sadly!

I just redeemed two codes through the website without any problems.

I redeemed some, after many tries, but one is gone for good!

Couldn't redeem Pankapu on Steam yesterday, tried several times...

Same. It says it has been redeemed, but then doesn't show. Now I'm wondering how many times this has happened in the past, and I've not noticed.

Having this exact same problem too. So glad I make sure to check it's been redeemed each time.

Yep same here ive tried many times today

From what I have read on reddit, people say it took them hours but the games eventually appeared, but still keeping an eye out either way.

they have to fix this cuz this is very annoying