Appears your SX80 was running TC7 software, and you upgraded to CE8 and later. Looking at the API guides, in order to use the POST method with CE8 and later, the commands/configurations will need to be contained within the HTTP body, instead of the URL. The HTTP header content-type should be set to "text/xml".
TC7 using GET, POST method:
http://<ip-address>/formputxml?xmldoc=<Configuration><SystemUnit><Name>newName</Name></SystemUnit></Configuration>
CE8 and later using POST method:
http://<ip-address>/putxml