Challenge: site migration goes wrong...

Hi there,
The thing I did succesfully before, does not work this time, and it's driving me nuts (after spending the whole weekend until now to get ik working....).
I recently moved a WP multisite on subdomains to a new host: downloading files, dumping db, and uploaded en imported on the new hosts servers. A bit of finetuning in the domainmapping and all was OK.
Now I do exactly the same with another site, from the same old host to the same new one.
The mainsite is OK, but the network-admin and the subsite are not visible.
All adjustments to wp-config.php lead to a 500 error message. I can backup and delete the MS tables and the make a network, but then my custom posts from CustomPress (configured networkwide, so settings not available on the mainsite) are gone.
All my tricks don't work.

Who can give his or her insights on this??

Thanks in advance!