... continued from &463/:
Actually, there is no resolution. My educated guess still is: The application is to heavy for www.infomaniak.ch. Reverse statement: www.infomaniak.ch is to weak for a 2007 RIA like Movable Type 4. I rolled back.
But nevertheless, I learned a lot: FTP throws away files - at least my client does. As soon as Movable Type's plugin folder misses, there are fewer Error 500s - at least one can revise existing entries (mt.cgi with parameter mode=view). One can publish new or revised entries by ignoring Error 500 following the Save button. As soon as one adds all plugins though, revisions and new entries cease to work at all. A workaround could be to regularly disable/ enable the plugins from the Movable Type 4 UI. The downside: All of you would see Error 500s when commenting. Not good. I leave it as it is, sticking with version 3 for now. Migration of the hand-crafted & the Red Fez templates and style sheets for Movable Type 3.35 would be an additional and considerable effort.
To close this rather technical topic - and that was the reason why I started the whole venture yesterday: I will need to and I want to move to the new platform to enter the mobile age. Have a look at the first steps - using described workarounds - as & the Red Fez optimized for iPhone!!