Reproduction steps:
1. Open the attached “IN-135832” project
2. In Project Settings → Player Settings -> Publishing Settings -> WebAssembly Language Features, enable "Enable Native C/C++ Multithreading"
3. Build the Web Player
4. Install the “local-web-server” package
5. Open the terminal inside the build folder and run “ws --https --cors.opener-policy same-origin --cors.embedder-policy require-corp“ to host a build with the required headers

Actual result: The webpage reloads multiple times or infinitely, sometimes it shows a black screen saying “A problem repeatedly occurred on “https://ip:8000/”.
Expected result: The webpage loads successfully on the first attempt

Reproducible with: 2023.1.0a15, 6000.0.71f1, 6000.3.12f1, 6000.4.1f1, 6000.5.0b1, 6000.6.0a1

Testing environment: macOS 26.3.1

Reproducible with these devices:
VLNQA00634 - iPhone 15 Plus (MU0Y3QN/A), CPU: t8120, OS: 26.3
By reporter - iPhone XS, OS: 18.7.5

Not reproducible with these devices:
VLNQA00626 - iPhone 16 (MYE73QN/A), CPU: Apple A18 Pro, OS: 18.6.2 (Player always opens on first attempt)

Couldn’t test with these devices:
VLNQA00491 - iPhone 14 Pro (MQ083PX/A), CPU: Apple A16 Bionic, OS: 16.0 (says that Safari 16.0 does not support multithreading)
VLNQA00346 - iPad Air (4th generation) (MYFR2HC/A), CPU: Apple A14 Bionic, OS: 26.1 (Some JavaScript error shows, but Player opens)

Note: Reproducible with Safari, Chrome and Firefox