RN214 Unable to upload Apps (from store or via upload on admin page)

Hello Netgear Community

Netgear NAS RN214 has been serving faithfully as a backup drive for various home devices but most usefully as a media store with Plex.  For reasons surpassing understanding, Plex stopped working and (perhaps stupidly) I thought a re-install might solve the problem.  

Plex duly uninstalled from the NAS, but...here is the problem:  I cannot reinstall it.  I have sourced what I believe to be the correct installation file from Plex but when I upload it on the admin apps page the admin web portal resets and when I log back in - no app is installed.  I am receiving no error messages.  

I have tried a few other apps just to see if it is Plex-specific, but the same thing happens with the others I have tried.  

Firmware is 6.10.6 - and all other signs are that the system is healthy - I just cannot upload any apps to it.

I am not a techno-wizard but willing to learn - and get my kids happy again as they are all a bit grumpy with the loss of the media library!  

Thanks in advance,

Marcus