Cannot login as admin from ONE particular place

Hello

I need help for a tricky situation for an access in admin mode which is not successful in
ONE particular situation.

Here is the case

– a laptop under windows XP
– a connexion to mysite/wp-admin/
– the normal login/password page
– the login/password couple is entered correctly
– a redirection is done

it works for any location where this PC is sitting except one !! Unfortunately this
is the home of the friend who needs to connect as admin.

So we made many tries.

Same PC, same browser, same login/password… if the PC is home, the connection failed, if the PC is
otherwhere, the connection is successful.

Any connection as simple visitor to browsing the site works fine. Only the connection
as special user (needing login/password) failed.

The hosting company (1and1) said to not be involved into any blocking scheme.

I have tried, removing plugins, reinstalling them…. no issue.

When the login failed, wordpress is displaying a white page with “site under maintenance”
which is not true.

for what circonstances, what reason, wordpress can display that a site is under maintenance ?

Any help is welcomed.

Thanks.