Yes. Galaxy never got this bug, it worked fine on monday, and still works now (while on browser it didn't work on either day for me).

Thank you.

Any update on the broken Wishlist filter?

No.

The more it happens, the more I'm convinced that it must be a Schrödinger's GOG-bear bug because, at the same time, the options work and they don't.

I noticed that when a browser tab is left open for some time without interaction, the options reappear, and they work for a while. Right now they keep working in one tab, while they are not available in another. When leaving a sales page where they worked and returning, the options are no longer available.

Using "games?discounted=true&hideDLCs=true&hideOwned=true" in the URL in a tab the options are not available leads to the display of GOG's 404 page.

The question is, who will win this battle, Dr. Schrödinger or Mr. GOG Bear?

For me the interesting thing, as some have noted, is that the filters are broken on the general games store and some of the special promos, but they still work (for me at least) on some other promos such as the Cozy Autumn and THQ Nordic promo sites. I am curious what causes that difference.

You are right. I left a tab open for a couple of hours and the options appeared. I then reloaded the tab and they disappeared.

Try reloading the page a couple of times and see if it changes anything (for better or worse).

I discovered a temporary fix for Firefox:
① Open the store page
② Press Shift + F9
③ Click on Local Storage in the left pane
④ Right-click on https://www.gog.com and Delete All
⑤ Close the panel with Shift + F9
⑥ Refresh the website once by pressing F5

Thanks for trying. That fix works exactly once for a single page reload. (At least it doesn't log out the user.)
The Firefox debugger lists persistent errors though, something about blocked cross-origin resource sharing (CORS) requests.

Impressive work, but I will not attempt it. I have no idea what deleting all of GOG's local storage would do and I am not adventurous enough to try.

Likely just clears the cache for the GOG site only.

It does the same as your browser's built-in 'Clear website data and cookies' function. It resets and clears all the data stored in the browser's localStorage object for gog.com. The difference of using the storage inspector and the localStorage.clear() method is that your session will not get interrupted when the process has finished deleting the data. The browser's 'Clear website data and cookies' is more thorough in that it clears all data, including cookies and logged-in sessions.

After you refresh the page with F5, the website will recreate some key-value pairs in localStorage, based on your current session and set preferences. There is no risk involved in using the described method, or else I would not have shared it (or used it myself). ;-)

Edit: I should also add that I don't want to encourage you to use this method because it's only a temporary solution. If you do choose to use it, you must be careful not to open the link to a game's store page in the same tab by clicking on it. Instead, you should use Shift+right-click to open the link in a new tab; otherwise, you would have to go back and repeat the process to be able to use the filters.
Yes, but where does this get us? I've been investigating it and found that, in my case, uBlock Origin's EasyList filters were blocking it. The extension isn't allowed to execute in private window sessions, and there were no blocked or missing certificates that could cause this error on our end. I am also in the process of investigating other possible sources contributing to this problem on our end, but so far, everything has come out inconclusive.

Do you have any ideas?

Thank you for explaining. I tried and indeed it works, though as you said, temporarily. Well done.

P.S. It did reset a thing or two beyond the bug:
- the catalogue display reverted to "tiles" from my default of "list", which wasn't too difficult to restore
- it cleared all my defaults for the "data grid" view GOG add-on, which was a bit of a pain as there were quite a few of them, but I survived

You are right, it seems those CORS errors were caused by uBlock Origin, but they are unrelated to the general problem.
B/c after disabling uBO, flushing local storage again and even re-logging into my account, the filters and labels on sales pages are still missing, as soon as I reload the page. :-(

No, I did not think CORS would be the problem either;

While conducting further testing, this time using Vivaldi, the internal debugger displayed an error message indicating that the Access Token Client was duplicated. This led me to a file called 'accessTokenClient.js,' which I added to the internal debugger's ignore list. After reloading the website, all filters became available. I was now able to leave the store page, reload it, log out of a session, and log back in without causing the filters to disappear.

Initially, more lines inside 'accessTokenClient.js' were marked as issues inside the file. However, now there is only one line present:

accessTokenClient.js
window.hasOwnProperty("gogAccessTokenClient") ? console.warn("Access Token Client is duplicated!") : window.gogAccessTokenClient = new o.AccessTokenClient

When accessing a game page, the debugger pauses and points to 'bundle.min.{...}.js:1' with the same line as above.

This issue is related to the duplication of the EventBus and points to 'gog-module-event-bus_min.js:1,' as you can see in the second and third screenshots, respectively.

While ignoring 'accessToken' in Vivaldi works without any issues, I don't know how to achieve the same result in Firefox, where the problem persists even after setting the debugger to ignore the file. Additionally, I'm unsure of how to block or remove the line inside the '.js' file and save the result permanently, so Firefox can't overwrite it. I hope that someone with more expertise can find a solution and share it.

Edit: It can still occur that the filters vanish in Vivaldi but Ctrl+R to reload the store page fixes it. This may be due to not setting a folder to store any overrides configured in the debugger, but I'm not entirely sure. Additionally, I believe that disabling the lazy loading of visited tabs in Firefox should bring me closer to a solution. The challenge here is that there doesn't appear to be an option for this in the settings. While 'about:config' does display some options, I am hesitant to make changes there when there might be alternative methods.