"Access denied" on Frodo MediaPortal plug-in

I have been having a frustrating time getting the TV functionality to run on XBMC Frodo (Distribution Candidate 3) on a remote (ethernet network) machine using the MediaPortal XBMC TV Server plug-in (version 1.2.3.118). The MediaPortal TV server is running on a Windows 7 64-bit machine and the client is a Windows 8 64-bit machine, although I have only just upgraded the client to Windows 8 and was having a range of problems trying to get this to work before I upgraded it as well.

The latest problem I am having is that when I select a TV channel to watch, it thinks for a while then I get a message: 'Access denied: \\SERVER\D$\MediaPortal shares\live3-0.ts.tsbuffer'.

I think I have configured the TV Server plug incorrectly in Media Portal (the available TV channels show up in XBMC okay and I get a message to say the EPG database is refreshed as well) including setting up Recording and Time Shifting shares for each of the TV cards I have installed on that machine, and I have made sure that I have shared the folder with "Everyone" giving full control. In XBMC I have installed the MediaPortal add on and I have configured it to point to the server's IP address. I have tried leaving the logged in user as "Guest" and also changing it to the name of the single user I have set up on that machine (with appropriate passwords for both). I am able to access the "mediaportal shares" folder that I established on the server from the client machine through Windows Explorer. I'm not sure if it's relevant, but the name of the recording/timeshift folder in the XBMC TV Server plug-in in MediaPortal on that server machine is "\\SERVER\MediaPortal shares", but the error I am getting on the client machine in the XBMC log shows that folder as "\\SERVER\D$\MediaPortal shares". The following is what I think is the appropriate part of the XBMC error log:

15:33:09 T:5952 NOTICE: Thread CDVDPlayer start, auto delete: false
15:33:09 T:5952 NOTICE: Creating InputStream
15:33:09 T:5952 DEBUG: PVRManager - PVR::CPVRManager::OpenLiveStream - opening live stream on channel 'TEN Digital'
15:33:09 T:5952 DEBUG: opening live stream for channel 'TEN Digital'
15:33:09 T:5952 DEBUG: AddOnLog: MediaPortal PVR Client: ->OpenLiveStream(uid=8)
15:33:10 T:4924 DEBUG: ------ Window Init (DialogBusy.xml) ------
15:33:11 T:5952 INFO: AddOnLog: MediaPortal PVR Client: Channel stream URL: rtsp://192.168.1.115/stream3.0, timeshift buffer: D:\MediaPortal shares\live3-0.ts.tsbuffer
15:33:11 T:5952 INFO: AddOnLog: MediaPortal PVR Client: Creating a new TsReader...
15:33:11 T:5952 NOTICE: AddOnLog: MediaPortal PVR Client: CTsReader::Open(D:\MediaPortal shares\live3-0.ts.tsbuffer)
15:33:11 T:5952 DEBUG: AddOnLog: MediaPortal PVR Client: Cannot access 'D:\MediaPortal shares\live3-0.ts.tsbuffer' directly. Assuming multiseat mode. Need to translate to UNC filename.
15:33:11 T:5952 INFO: AddOnLog: MediaPortal PVR Client: CTsReader:TranslatePath D:\MediaPortal shares\live3-0.ts.tsbuffer -> \\SERVER\D$\MediaPortal shares\live3-0.ts.tsbuffer
15:33:19 T:4924 DEBUG: ------ Window Deinit (Pointer.xml) ------
15:33:31 T:5952 ERROR: AddOnLog: MediaPortal PVR Client: Access denied on \\SERVER\D$\MediaPortal shares\live3-0.ts.tsbuffer. Check share access rights for user 'Mark' or connect as a different user using the Explorer.
15:33:31 T:5952 DEBUG: CAddonCallbacksAddon - ADDON::CAddonCallbacksAddon::QueueNotification - MediaPortal PVR Client - Error Message : 'Access denied: \\SERVER\D$\MediaPortal shares\live3-0.ts.tsbuffer'
15:33:31 T:5952 ERROR: CDVDPlayer::OpenInputStream - error opening [pvr://channels/tv/All TV channels/5.pvr]
15:33:31 T:5952 NOTICE: CDVDPlayer::OnExit()
15:33:31 T:5952 NOTICE: CDVDPlayer::OnExit() deleting input stream