Microsoft recently launched a security patch for Internet Explorer which requires a 1024 bits long key. Originally when the EM Database Control (DB Console) was installed the self generated key length is 512 bits.
The IE therefore reports a problem with the web site (EM) certificate.
Oracle has recently published a note on My Oracle Support (MOS) under ID 1498203.1
"Log In To Grid Control Console 10g or DB Control 10g and 11g within Internet Explorer 7, 8 or 9 fails with "Internet Explorer cannot display this page" or with Security Certificate Error".
Actually one has to patch the EM in order to be able to increase the key length to 1024 bits. The command which should be used is:
emctl secure dbconsole -key_strength 1024 -reset
However, without first applying the Patch 14503114 one will get error that the parameter -key_strength is not supported. So in order to resolve the problem patch has to be applied.
The other possible workaround is to use another browser which is satisfied with 512 bits long key. Google Chrome and Firefox work just perfectly.