WebSockets stop working on year-old project

I have ESP8266 project using web-sockets (library by Markus Sattler) to control device, the project was definitely working around year (or so) ago. Since that there were no changes, unless standard updates for Windows (now is Windows 11 Pro 24H2), Arduino IDE (now is 2.3.4), WebSockets library (now is 2.6.1), and some other libraries which are probably not relevant. Have just tried to load this project now and have find out that it is compiled, but is not working, I cannot connect to websocket-server from the Internet browser. Project is relatively complex, a lot of other libraries are involved, but seems like the rest of functionality is working OK.

I have just started to troubleshoot what is going on there, and decide to ask almighty all-known people here - are there any critical changes in WebSockets library or in Windows browser's support of web-sockets, which may turn good working project into the non-working state?

You didn't mention version of the ESP8266 core. I've had occasions where code would work on my laptop, but not on my desktop. After some time I found that the only difference was the core and using an older version of the core would make the sketch working.

(After that I could dig a bit deeper into the differences to find out that the older core didn't mind using timing functions in a constructor and the newer core did...)

You didn't mention version of the ESP8266 core.

Yes, agreed... ESP8266 core version currently is 3.1.2.
But, I believed, it has not been updated since my project was known as working.
I believe the ESP8266 core has no update for about two years...

So can you connect to websocket server using other medium like postman or any other apps ?

Five years ago I used web-sockets to control a D1 mini robot cart. I haven't tried to recompile that project ever since, so it might give issues now.

It used a virtual joystick provided via locally served Javascript to control the ESP8266 via the same arduinoWebSockets library (author aka @Links2004); see also this Arduino library reference.

Within that library there are some examples. Do those still work?
(Unfortunately I currently don't have time to dig into this myself...)

Ok, seems like I've got the answer, problem was in newer version of WebSockets library by Markus Sattler, latest version 2.6.1 gives me such problem. Have downgraded to the 2.4.0 - everything works now. Have not yet tried versions in between so do not know which is last good version, I've specifically downgraded to 2.4.0 because I have checked my six-month-old backup and get version from there.

Okay, that's good to know.
I saw some things mentioned on Github issues that might be related to this issue, but didn't have time to look any further.