Comparing activities no longer works

Still works for me on multiple different browsers. Did you check the Console in browser's DevTools for errors?

You could try it with an alternative internet connection. Sometimes, CloudFlare blocks the access to some of Garmin's services, and changing the IP address, or the access provider, can help.

Same issue for me. Tried Safari on MacOS 15 with and without Private Relay via two different internet providers, Safari on IOS, Edge on Windows 11.

There's currently a problem in the Connect website activity details where the chart order isn't saved for running activities if you don't have a step speed loss chart. (This problem affects me and others.)

So I wouldn't be surprised if something similar is happening here. Meaning Garmin may have made another change that breaks things for some users, but they aren't aware of it.

Having said that, I don't have the same problem.

As trux suggested, it might be helpful to check your browser console for errors when this happens. I recommend using Chrome or Firefox, but Safari should also work

1) Open an activity comparison that fails

2) In the same tab, open your browser's dev tools:

Windows: CTRL-SHIFT-I

Mac: CMD-OPTION-I

3) Switch to the dev tools console tab and look for messages near the bottom, especially messages highlighted in yellow (warnings) or red (errors). I noticed that activity comparisons print out a lot of log noise in the form of "Missing key:..." messages, but I think these are harmless. (I see a lot of them and my comparison still works.)

What you want to look for is a red message at the bottom of the output.

If you can find that, it might give us a clue as to what the problem is.

I've now taken a look at the error message in the console.

The following error message appears several times:
XML parsing error: not well-formed

And these two once:
TypeError: can't access property "dataKey", e is undefined

Uncaught TypeError: can't access property "dataKey", e is undefined

Otherwise, there are also several warnings:
Missing key and source map error: request failed with status 403

This time I used the Firefox browser.

But it doesn't work in Chrome or Brave either.

I've also changed the DNS provider or tried using cellular on Android. That didn't help either.

Interestingly, I can compare some older activities. A white page appears first, but pressing F5 (refresh) 
loads the page.
Very strange, all of this...

The last console error message I am receiving: "Source Map-Ladefehler" with 16 details "Failed to load resource: the server responded with a status of 403 ()". If I am trying to open a URL in the details I am redirected to a page stating access is blocked by Cloudflare.

JR said:

The last console error message I am receiving: "Source Map-Ladefehler" with 16 details "Failed to load resource: the server responded with a status of 403 ()".

This one is irrelevant. It just tells that the website does not offer Source Maps, for their encoded/minimized javascrip files. And that's normal.

I use Cloudflare as my DNS.

I changed it back to the default DNS.

But, as I said, that unfortunately doesn't change anything.

But in the Brave browser (Chromium-based), I was able to get another error message:
{"errorType":"authentication_failure","errorMessage":
"Requested failed with InsufficientAuthenticationException",
"errorId":"fd7aade3-b693-4c63-9bfa-4dca1cd8f0ff"}

So, an authentication problem?

Why is that?

The Network tab may reveal some more clues. Which URLs come up red? And what the Response tell when you click on them?

I was able to extract the following from the Network Analysis tab:

Status
429
Version HTTP/3
Transferred 807 B (198 B size)
Referrer Policy Origin
DNS Resolution System
Blocked
Enhanced Tracking Protection
The address matches a known tracking element
and would be blocked if blocking page elements for tracking was active.

I've already disabled everything in Firefox or allowed unsafe ones to avoid conflicts. 
I've also disabled all add-ons to avoid conflicts.
I can't find anything else that can be disabled. 
But apparently the browser still thinks it has to block something.
But whether it's really related to that is questionable.

Status 429 is interesting, it says "Too Many Requests."
Well, I've been trying all the time...

So, I'm not really getting anywhere...

Try it from another computer, preferably on a different location (i.e. from your job, school, or a public place).

Also, you do not post sufficient details, like for example the exact URL that gets blocked. that would help analyzing the issue. Rather post some screenshots, like the following ones: