2015-12-11 10:27:21.879-05:00 [HOST:lx-pmmstgy-p06p][PID:5211][THR:1152923968][Object Server][Error][0x80041A09] Invalid Expression.
2015-12-11 10:27:18.792-05:00 [HOST:lx-pmmstgy-p07p][PID:8414][THR:1813031232][Object Server][Error][0x80041005] Index out of range in method 'LeanCollection::ItemNS'
2015-12-11 10:27:07.439-05:00 [HOST:lx-pmmstgy-p05p][PID:18853][THR:3383707968][Object Server][Error][0x80041105] The object with the given identifier is not an object of the expected type.
2015-12-11 10:27:05.029-05:00 [HOST:lx-pmmstgy-p08p][PID:8380][THR:3240225088][Authentication Server][Error][0x80043F69] An error occurred during authentication. Please contact your administrator: LDAP Server error (19): Constraint violation.
2015-12-11 10:26:43.599-05:00 [HOST:lx-pmmstgy-p07p][PID:8414][THR:2939128128][Network Classes][Error] Closing idle connetion. socket = 511
Your regex was only extracting text after Error because that's what you told rex to look for - "[Error]" then the field.
Assuming you always have three sets of brackets before the one you want, this regex should work for you.
--- If this reply helps you, an upvote would be appreciated.