PuSHPress caused "unexpected output" problem on subsequent plugin activations

On my DreamHost shared server, PuSHPress caused a problem where every time I activated another plugin, it “generated 311 characters of unexpected output during activation.” (See this WordPress StackExchange thread for more.)

I used System Information to trace the problem to the activated_plugin hook, which was attached to lacandsnw_pushpresscheck even months after I installed PuSHPress. Deactivating PuSHPress stopped the unexpected outputs.

Let me know what other information is needed to diagnose and fix the problem. I have disabled PuSHPress for now and am using only PubSubHubbub until the issue is fixed.