I've gone to verify the theme to one of our larger applications, and I've received the same error @"Alli Pierre Yotti" and @"sect55" received in this archived post:
ORA-00001: unique constraint (APEX_050100.WWV_FLOW_PAGE_PLUG_IDX2) violated

This particular application has some original templates, which come up as "Not subscribed"

I've successfully verified other applications that also have their own templates, so I'm not convinced this is the cause, particularly since the error relates to a unique index on WWV_FLOW_PAGE_PLUG_TEMPLATES(FLOW_ID, PAGE_PLUG_TEMPLATE_NAME, THEME_ID).
I'm not sure what I can do to mitigate this?
This is on APEX 5.1.3
Scott