Subscriptions has been working fine for several weeks, but all of a sudden, a bunch of my subscriptions slipped into "Past Due" status. I tried to manually enter the module action URL in my browser (they have it programmed to display a status of what happens as it attempts to renew any past due subscriptions), but after outputting a few rows it reloads, this time using the EE Error template (user message template), and refuses to continue.
The "error" was something about a missing {/if} or errant } or something. This didn't make any sense because the action url shouldn't invoke the template parsing engine.
How could something in the Subscriptions update process trigger an error in the template parsing engine?