We checked Online Maps with your settings, and can not reproduce the problem.
Still no solutions for similar problems in the Internet.

Please send your scene (as a package) to us (support@infinity-code.com).
Maybe the problem will show itself.

Unity 5.5.0f3 64bit
Source : Online
Provider : ArcGIS(Esri)
Type : WorldImagery
Zoom : 13
Width : 4096
Height : 4096
Size : x 1024 y 1024
elevation enabled
user control disabled
allow zoom disabled
no markers

Cooking::cook/createTriangleMesh: user-provided triangle mesh descriptor is invalid!
UnityEngine.MeshCollider:set_sharedMesh(Mesh)
OnlineMapsTileSetControl:UpdateMapMesh() (at Assets/Infinity Code/Online maps/Scripts/Controls/OnlineMapsTileSetControl.cs:1445)
OnlineMapsTileSetControl:UpdateControl() (at Assets/Infinity Code/Online maps/Scripts/Controls/OnlineMapsTileSetControl.cs:1201)
OnlineMapsTileSetControl:OnElevationRequestComplete(String) (at Assets/Infinity Code/Online maps/Scripts/Controls/OnlineMapsTileSetControl.cs:989)
OnlineMapsTextWebService:OnRequestComplete(OnlineMapsWWW) (at Assets/Infinity Code/Online maps/Scripts/WebServices/Base/OnlineMapsTextWebService.cs:56)
OnlineMapsWWW:Finish() (at Assets/Infinity Code/Online maps/Scripts/ExtraTypes/OnlineMapsWWW.cs:196)
<WaitResponse>c__Iterator0:MoveNext() (at Assets/Infinity Code/Online maps/Scripts/ExtraTypes/OnlineMapsWWW.cs:308)
UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)

Unfortunately, I do not know what the problem is in your case.
I found some similar problems on the internet, but all of these problems do not have any answers.
Try to use a different type of Collider (Online Maps Tileset Control / Collider Type). Maybe it will help.

The version of the Unity editor is 5.5.0f3. We are using version 2.5.1.4 of your tool.

To reproduce this, all I am doing is zooming in or out using the mouse scroll wheel. It seems to happen when zooming faster, rather than slower. It happens frequently if zooming quickly.

I can send you a screenshot of our OnlineMaps settings if that helps.

Here are some of the main settings:

Source - Online
Provider - ArcGIS (Esri)
Type - WorldImagery
Zoom - 4
Target - Tileset
Width - 4096
Height - 4096
Size 3000 x 3000

Zooming enabled
Zoom Mode Target
Smooth zoom enabled

Allow camera control enabled
Camera distance 1460

Use elevation enabled.

I can disable error pausing in Unity, and it seems like we can continue after the error okay, but it would be nice to be able to have that enabled while developing to pause when other errors occur.

Thanks!

This is the first report about such problem.
Please specify version of Unity Editor, and give more detailed instructions on how to reproduce the problem.

When zooming with OnlineMaps I have often been getting an error:

Cooking::cook/createTriangleMesh: user-provided triangle mesh descriptor is invalid!
UnityEngine.MeshCollider:set_sharedMesh(Mesh)

Further details:

OnlineMapsTileSetControl:UpdateMapMesh() (at Assets/Infinity Code/Online maps/Scripts/Controls/OnlineMapsTileSetControl.cs:1411)
OnlineMapsTileSetControl:UpdateControl() (at Assets/Infinity Code/Online maps/Scripts/Controls/OnlineMapsTileSetControl.cs:1177)
OnlineMapsTileSetControl:OnElevationRequestComplete(String) (at Assets/Infinity Code/Online maps/Scripts/Controls/OnlineMapsTileSetControl.cs:971)
OnlineMapsTextWebService:OnRequestComplete(OnlineMapsWWW) (at Assets/Infinity Code/Online maps/Scripts/WebServices/Base/OnlineMapsTextWebService.cs:56)
OnlineMapsWWW:Finish() (at Assets/Infinity Code/Online maps/Scripts/ExtraTypes/OnlineMapsWWW.cs:196)
<WaitResponse>c__Iterator0:MoveNext() (at Assets/Infinity Code/Online maps/Scripts/ExtraTypes/OnlineMapsWWW.cs:308)
UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)

It seems like this error has mostly been occuring since we updated to the beta 2.5 version, and still now with the current 2.5.1.4 version. Perhaps it is related to our maps settings? Or is this a known error?

The error will pause the Unity editor, and it seems like we can just unpause and continue and it runs okay, so maybe this won't be an issue after building the project, but it happens often enough that it gets in the way of testing and using the application.