Your query is entitled "Unable to update" but you mention that you have "have just completed a clean install" so I'm not sure if you are trying to upgrade an existing Showkase site or creating a new site from scratch.
Either way, the first thing I would do is re-upload the latest core Showkase files to your web server in case something happened during the initial upload which has somehow resulted in a corrupt or missing file.
Also, if you are able to, try running 'System -> Setup' from the admin pages. Also, try 'System -> Repairs'.
If this does not work, then try opening the 'showkase/_showkase/showkaseconfig.ini' file in a plain text editor and check that all paths to your Showkase site are correct. (The file is small and has only three paths to check.)
You could also try checking the permissions of all Showkase files and folders on your web server. Default permissions of 644 for files and 755 for folders should hopefully work fine but please see this web page for more information on permissions.
Even though you say that your web server is 100% compatible, I would also recommend that you try installing and running the Showkase Server Compatibility Test, just in case your web host has made any changes recently which are contributing to your problem. The Showkase Server Compatibility Test can be downloaded from here.
Another thing you could try is to go through the lost password procedure outlined in this FAQ:
I lost my password, how can I reset it?
Hopefully one of the suggestions above will help.