Hi Peter,
TestComplete 8.70 doesn't recognize your tested application as Open. First of all, please make sure that the .NET Open Application Support plug-in is installed and enabled in TestComplete ("File | Install Extension..."). Also, a possible cause of the problem is that some complex actions which take long to be completed are performed in the application's GUI threads when the application is started (the application's main window may become blank or unresponsive in such cases). If so, try increasing the value of the "Method invoke timeout" option (set it to more than 30000) described in the
"Project Properties - General Open Applications Options" article.
Does this help?