This issue was caused by a call to Parallel.For in a user script in the Curvy package. Unfortunately, C# multithreading is not supported yet for WebGL, although we are looking at adding support for this in the future.
If you want to preserve this behavior for non WebGL builds, you can conditionally compile this logic using #if UNITY_WEBGL to control whether a parallel or standard for loop is used. Lastly, please refer to the Unity WebGL documentation for more details regarding constraints that are involved when targeting WebGL builds: https://docs.unity3d.com/Manual/webgl.html.