Thread hangs inside HTML Tidy function call, but only if being called from another process

Question

Having the following application invocation chain, my application hangs:

User starts .NET App1 → starts process .NET App2 → calls tidyParseSing() → hangs in there.

Having the following application chain, everything works:

User starts .NET App2 → calls tidyParseSing() → works.

So basically when my one app calls my second app which calls some HTML Tidy functions (through this PInvoke wrapper) in a background thread, the HTML Tidy functions stall.

This looks completely weird to me. I can only think of being completely on the wrong track, but still I have no clue on what's going on here.

So my question is:

Is there any possible difference between calling a PInvoke function from a process that the user started and a process that was started from another .NET process?

More information:

Both of my applications are .NET 4 x86 applications having the following app.config files:

<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0" />
  </startup>

  <runtime>
    <loadFromRemoteSources enabled="true" />
    <NetFx40_LegacySecurityPolicy enabled="false" />
    <trust legacyCasModel="false" level="Full" />
  </runtime>
</configuration>