Just putting this out here as a heads up for others who keep their production OEM installations on the bleeding edge with each new patch release.
I experienced a problem after installing the October 2017 DB plugin bundle patch 26955424 on my agent targets. It is not clear if the problem was due to the DB plugin itself, or the new 13.9.2.1.0 release of OPatch installed during the plugin deployment.
While the patch plan to apply 26955424 was still executing, all of my "Systems Infrastructure Server" targets went into "Status Pending" from up, and several of my OEM internal targets (like the BIP and EMGC application deployments) began to show as down. They weren't down, but OEM lost the ability to monitor them. I have an open SR on this issue. If like me you applied this to your central agent and had internal targets show as down, I recovered from that by restoring my central agent from filesystem backup then doing an agent resync and clearstate.
To make a long story short there's something broken with either the newest OPatch or the October 2017 that causes monitoring issues for OEM targets, resulting in various errors referencing "oracle/xml/parser/v2/DOMParser" from agent emctl commands and target monitoring. Bugs 27102554 (public) and 27099568 (not currently public) are tracking this issue. Despite the targets showing as down or status pending, the rest of my OEM environment appears functional (databases monitoring continues, scheduled jobs continue running as expected), so if you get into this situation, don't panic.