I only have access to the XP machine right now. I did some more step by step poking and it looks like "Reset Log" doesn't actually generate the new file/properly clean up the old file until you do a subset of things. Opening menu dialogs (About JRiver, Logging, Eula, etc) normally generates log file entries but doesn't trigger "fixing" the log file. Clicking on left pane items (Smartlists, playing now, etc) does cause MC to "fix" the log file. Once the log file has been reset and fixed in a session, it seems to always work after that.
Steps I did:
Unistalled JRiver Audio Driver from Hardware control panel.
Open MC20.21.
Help->Logging (confirm it is turned on)
Navigate to Application Data\J River\Media Center 20
Open Log.txt in notepad. It contains logging information. It is 49kb.
Click "Reset Log" in MC.
Click OK in resulting dialog.
Return to Explorer. Log.txt still exists, still shows 49kb.
Refresh folder, no change.
Try to open Log.txt, access denied.
Close logging window. Do some clicking in MC20.
Now Log.txt changes size and is openable.
Now I'm no longer able to generate access denied using original steps.
Clicking Reset Log at this point always sets the size back to 0kb.
Close MC.
Open MC.
It updates itself to .47 without asking.
Now I have Log.txt and Previous Log.txt
Can open Log.txt
Click Reset Log
Click Okay
Previous Log.txt is gone, Log.txt is 47kb and access denied in notepad and in "View Current Log".
Access menu items (View Eula, Benchmark)
Log.txt still 47kb and access denied.
Click Smartlists in left menu
Log.txt resets and is openable
Click Reset Log
Click Okay
Log.txt resets and is openable
Log after "good" reset:
Media Center; Version: 20.0.47; Types: 2147483647
0000000: 3152: General: Starting logging: Date: 12/10/2014 12:18 pm
0000000: 3152: General: Log Reset: Logging reset
0000812: 3152: General: CMCResourceHelper::GetIsModalPopupShowing: Main window disabled
0001322: 3152: General: CMCResourceHelper::GetIsModalPopupShowing: Main window disabled
0001322: 3152: General: RunProgram: Start
0001322: 3152: General: RunProgram: Filename: C:\Documents and Settings\XPMUser\Application Data\J River\Media Center 20\Log.txt / Parameters:
0001322: 3152: General: RunProgram: Performing ShellExecute...
0001422: 3152: General: RunProgram: Running process...
0001422: 3152: General: RunProgram: Finish (100 ms)
Log after "bad" reset (notice no Log Reset or Starting logging):
Media Center; Version: 20.0.47; Types: 2147483647
0020570: 2628: General: JRWebWnd::OnTerminate: Closing callback server
0020570: 2628: General: JRIpcServerThreaded::ServerStop: Start
0020570: 2628: General: JRIpcServerThreaded::ServerStop: Canceling thread
0020570: 2628: General: JRIpcServerThreaded::ServerStop: Canceling pending run
0020570: 2628: General: JRIpcServerThreaded::ServerStop: Stopping thread
0020580: 888: General: JRIpcServerThreaded::Thread: Thread finishing (cancel: 1; errors: 1)
0020580: 888: General: JRIpcServerThreaded::Thread: Finish (28411 ms)
0020590: 2628: General: JRIpcServerThreaded::ServerStop: Canceling response threads
0020590: 2628: General: JRIpcServerThreaded::ServerStop: Deleting response threads
0020590: 2628: General: JRIpcServerThreaded::ServerStop: Closing notification window
0020590: 2628: General: JRIpcServerThreaded::ServerStop: Finish (20 ms)
0020590: 2628: General: JRWebWnd::OnTerminate: Closing window
0020590: 2628: General: JRWebWnd::OnTerminate: Finish (90 ms)
0022162: 2628: General: CMCResourceHelper::GetIsModalPopupShowing: Menu showing
0022473: 2628: General: CMCUICore::SystemPowerManager_UpdateActions: Start
0022473: 2628: General: CMCUICore::SystemPowerManager_UpdateActions: UI Mode: -999; Playing: 0
0022473: 2628: General: CMCUICore::SystemPowerManager_UpdateActions: Finish (result: 0) (0 ms)
0023124: 2628: General: CMCResourceHelper::GetIsModalPopupShowing: Menu showing
0023724: 2628: General: CMCResourceHelper::GetIsModalPopupShowing: Main window disabled
0023724: 2628: General: RunProgram: Start
0023724: 2628: General: RunProgram: Filename: C:\Documents and Settings\XPMUser\Application Data\J River\Media Center 20\Log.txt / Parameters:
0023724: 2628: General: RunProgram: Performing ShellExecute...
0023895: 2628: General: RunProgram: Running process...
0023895: 2628: General: RunProgram: Finish (171 ms)