If you are using colorbox module then you have to stay with the 7.14 or older versions because as its clarified in colorbox page Form support is removed:

If you absolutely want to continue using the form support you need to stay with 7.x-1.x.

More specific the 7.14 and older versions because the 7.15 nor supports it (documentation misunderstanding).
In the meanwhile i can think of two work arounds:

  1. Check the current colorbox version in line 1409 (newsletter.module)
  2. Make use of the colorbox module in another way, if exists

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ParisLiakos’s picture

ParisLiakos CreditAttribution: ParisLiakos commented
Issue summary: View changes
Status: Active » Fixed

I just dropped colorbox support. If someone needs it a separate module should be made and figure a way to solve this problem

Status: Fixed » Closed (fixed)

Automatically closed - issue fixed for 2 weeks with no activity.