well, 3 hours later...
this was the workflow:
1. remove everything, try installing a vanilla setup - all ok
2. remove everything, try moving a local site zipped file to the server then extracting,
3. modify cache folder to 777
4. modify config.inc.php in /core/config to use server paths as specified in Bob’s faq -
http://bobsguides.com/modx-installation-faq.html
5. try logging into the manager - ALL OK!