Your comments

I had attempted earlier to repro in an isolated test project but was unable to. We'll have to leave this one unresolved.

Not sure if this is relevant, but it says "Assembly Info" Targets .NET 4.x

Client/Packages/manifest.json

{
"dependencies": {
"com.unity.2d.sprite": "1.0.0",
"com.unity.2d.tilemap": "1.0.0",
"com.unity.addressables": "1.1.7",
"com.unity.analytics": "3.3.2",
"com.unity.ext.nunit": "1.0.0",
"com.unity.ide.rider": "1.0.8",
"com.unity.ide.vscode": "1.0.7",
"com.unity.package-manager-ui": "2.2.0",
"com.unity.playablegraph-visualizer": "0.2.1-preview.3",
"com.unity.postprocessing": "2.1.7",
"com.unity.probuilder": "4.0.5",
"com.unity.progrids": "3.0.3-preview.4",
"com.unity.test-framework": "1.0.13",
"com.unity.textmeshpro": "2.0.1",
"com.unity.timeline": "1.1.0",
"com.unity.ugui": "1.0.0",
"com.unity.modules.ai": "1.0.0",
"com.unity.modules.androidjni": "1.0.0",
"com.unity.modules.animation": "1.0.0",
"com.unity.modules.assetbundle": "1.0.0",
"com.unity.modules.audio": "1.0.0",
"com.unity.modules.cloth": "1.0.0",
"com.unity.modules.director": "1.0.0",
"com.unity.modules.imageconversion": "1.0.0",
"com.unity.modules.imgui": "1.0.0",
"com.unity.modules.jsonserialize": "1.0.0",
"com.unity.modules.particlesystem": "1.0.0",
"com.unity.modules.physics": "1.0.0",
"com.unity.modules.physics2d": "1.0.0",
"com.unity.modules.screencapture": "1.0.0",
"com.unity.modules.terrain": "1.0.0",
"com.unity.modules.terrainphysics": "1.0.0",
"com.unity.modules.tilemap": "1.0.0",
"com.unity.modules.ui": "1.0.0",
"com.unity.modules.uielements": "1.0.0",
"com.unity.modules.umbra": "1.0.0",
"com.unity.modules.unityanalytics": "1.0.0",
"com.unity.modules.unitywebrequest": "1.0.0",
"com.unity.modules.unitywebrequestassetbundle": "1.0.0",
"com.unity.modules.unitywebrequestaudio": "1.0.0",
"com.unity.modules.unitywebrequesttexture": "1.0.0",
"com.unity.modules.unitywebrequestwww": "1.0.0",
"com.unity.modules.vehicles": "1.0.0",
"com.unity.modules.video": "1.0.0",
"com.unity.modules.vr": "1.0.0",
"com.unity.modules.wind": "1.0.0",
"com.unity.modules.xr": "1.0.0"
}
}

I don't think the problem is the DLL, as I exported the DLL to a new project and it worked. Here are the settings regardless.

Scripting and API compatibility level may have been greyed out due to settings not being checked out at the time of the first screenshot?

I tried the DLL in a new project and it worked fine too. I'm not sure what the problem is, which is unfortunate since Peek is such a huge productivity booster.

I tried Peek in a new project using the same Unity version and target and it worked. Peek failing in my main project could be due to the use of a DLL, which contains about half the code for the project.