SQL state S1000 in SQLConnect (but not all the time)

I have a CentOS6 webserver and a database server (MS SQL Server 2012). When I use the website everything is going fine except that once an hour or so I get a can't connect error in my php code. I looked in the ssl_error_log and found the following error at the same time I got the error: PHP Warning: odbc_connect(): SQL error: [unixODBC][FreeTDS][SQL Server]Unable to connect to data source, SQL state S1000 in SQLConnect Can anyone help me solve this strange problem? I'm not sure if it's a problem with the sql server or the webserver. I can't find any problems in the event viewer on the sql server.