Migation 2.5.28 to 3.4.1 fails - Joomla! Forum - community, help and support

i not joomla novice, drives me nuts...

trying migrate site (size 4gb, db 26mb)
i have stopped counting failed migrations... trying different alternatives...
it stops @ 86% of migration everytime, on url
http://mydomain.net/administrator/index ... e.finalise
with following error message:
"internal server error
the server encountered internal error or misconfiguration , unable complete request.
please contact server administrator inform of time error occurred , of might have done may have caused error.
more information error may available in server error log."

i use own vps plesk 12.0.18
26gb free disk space, 4gb ram
ubuntu 12.04.1 lts
php version 5.6.5 (have tested 5.3.10 bad result)
database version 5.5.28-0ubuntu0.12.04.3
database collation utf8_general_ci

php info:
max_execution_time 3600 seconds
max_input_time 360 seconds
memory_limit 256m
post_max_size 256m
upload_max_filesize 256m
open_basedir = /var/www/vhosts/mydomain.net/:/tmp/
upload_tmp_dir = /var/www/vhosts/mydomain.net/httpdocs/tmp
session.save_path = no value (default)

the server error.log ends these lines:
[sun may 17 17:29:23 2015] [error] [client xxx.xxx.xxx.xxx] file not exist: /var/www/vhosts/mydomain.net/httpdocs/installation, referer: http://www.mydomain.net/installation/in ... w=finalise
[sun may 17 18:05:05 2015] [error] [client xxx.xxx.xxx.xxx] premature end of script headers: index.php, referer: http://www.mydomain.net/administrator/i ... te.install
pls note tries search installation folder?! wht?

the /log/joomla_update.php ends on line 104 is:
2015-05-17t16:05:05+00:00 info xxx.xxx.xxx.xxx update ran query file 3.1.0. query text: -- -- table structure table `#__content_types` -- create table if not exist.

two more strange things:
1. site, installed same akeeba package installs fine om windows xampp (php 5.5.19)
2. (smaller) site installs fune on live test server.

any new suggestions welcome, i´m out of ideas now.

what if try migration on xampp , upload when you're done? way you'll able adjust server migration script (instead of other way around).

also, sure you're not running out of memory , you're not running out of disk space (also keep in mind hosts allocate specific amount of disks databases).