Since 0.59 patch, the admin logfiles (adm) are overwritten each time the server restarts. So instead of appending new entries after a (regular or manual) restart, a new, blank logfile is created and replaces the old one. This renders the functionality of the logfiles for finding hackers pretty useless.
We have already contacted our GSP Fragnet and the problem said it is not on their side. I also contacted Bohemia's David Kominek and he also said I should place a ticket.
Our server info is
37.187.72.43:2802
GSP is Fragnet
This problem appears to be only with our server as asking around other admins I found out that other servers with other GSPs do not have this problem.