Problem: HP Web Jetadmin reports a connection error with an HP LaserJet 600 series printer, even when you verify that the web administration pages are available.
Solution: Replace the MD5 security certificate on the printer with SHA1 certificate. Log in to the printer on the web admin pages if needed. Go to Networking > Authorization > JetDirect Certificate. Click View and you will see the value of the Signature algorithm field begins with md5, which confirms this issue. Click OK and choose "Configure" (for JetDirect Certificate). Choose Self-Signed and at the following screen click Finish.