I have exactly the same problem and same situation. Going from 1.5.1 to 1.5.5
The procedure I use is
install a fresh version of 1.5.5a
delete all the tables the db 1.5.5 created during install
copy the db 1.5.1 into the emptied 1.5.5
run zc_install to update the db back to 1.5.5
copy the italian languages files from 1.5.1 to the freshly installed 1.5.5
install all the plugins without any problem
install login as customer and it fails
(in my live version 1.5.1 login as customer works well)
I went trough the whole customers.php file and I can not understand why the link is built as follows
http://www.mystore.it/myadmin/customers.php?vcheck=yesindex.php?main_page=login_as_customer
which is obviously wrong becaue it should be
http://www.mystore.it/index.php?main...in_as_customer
Any help would be appreciated.
Thanks
enzo