TOPIC:

I think RC1 is not a good choice (however I never tried it).

First question: Which Windows are you using?

Let's try this simple path:
1. Uninstall any version you've got.
2. Install official (vanilla) OpenTTD 1.11.11 x64. Check it.
3. Overwrite installation with BTPro patch (official version from the web page). Check it.
4. Use my build (overwrite exe file). Check it.

"Check it" in points above means: try to run it, check if it works/crashes. And let me know.

There is a bug in the mainline that does exactly what you describe. Follows a workaround that you can try: github.com/OpenTTD/OpenTTD/issues/8967#issuecomment-819775499

@Shinrei: I have no time now to read whole thread mentioned by Myhorta, but maybe your problem is related to OpenGL issues mentioned there. If you want I can build you the version with OpenGL disabled. Let me know.

Somewhere in the thread posted by Myhorta I have found the following link:
www.openttd.org/downloads/openttd-nightlies/latest.html
From this link I downloaded openttd-20210429-master-g0b460bf4a1-windows-win64.zip. Unzipped it. Then I copied files in unzipped folder to S:\Game\OpenTTD replacing similarly named files. Runned openttd.exe and got the message:
Error with video settings ...
... no compatible GPU found. Hardware acceleration disabled
I am running Windows 7 x64 on laptop Dell Inspiron n5110 having processor integrated graphics card in Intel Core i7-2670QM as well as NVIDIA GeForce GT 525M on separate chip.

After that, I copied openttd.exe, lang and newgrf folders from BTPro_Client_1.11.1_1111A_64bit to S:\Game\OpenTTD. Then I runned openttd.exe. Now I can excess BTPro servers. Everything seems to be working after this manipulations.
Myhorta, please, accept my deepest thanks for directing to correct solution.