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Best answer

We hit an error adding your new account "authentication failed: Template string must be a string" Mailerlite Classic

  • November 12, 2025
  • 3 replies
  • 21 views

Hi

Mailerlite Classic has been working fine up until the email structure that I pull data from changed and I needed to update the zap. Now I can not get mailerlite classic to connect.

I have tried a few different browsers and still get the same error.
Have also regenerated a new API key in mailerlite too.

The error im getting is:
 



 

Ive also tried pasting in as plain text to the key.

Also, this error is shown too.
 



Any help would be appreciated.



Thanks

Stu

Best answer by SamB

Hi ​@StuJ, I’m pleased to share that the bug has been marked as fixed! 🎉

Appears to have been an issue with a recent Platform upgrade so the team have reverted the changes associated with the update, which should have resolved it.

Can you try adding your MailerLite account again and confirm whether it works now?

3 replies

Troy Tessalone
Zapier Orchestrator & Solution Partner
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  • Zapier Orchestrator & Solution Partner
  • November 12, 2025

Hi ​@StuJ 

Help links for using MailerLite Classic in Zaps: https://zapier.com/apps/mailerlite-classic/integrations#help

Try reaching out to MailerList Classic Support for help related to authentication.

Or try opening a ticket with Zapier Support: https://zapier.com/app/get-help


SamB
Community Manager
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  • Community Manager
  • November 13, 2025

Hey there ​@StuJ 👋

I’m so sorry to hear that you’re running into errors when attempting to connect your Mailerlite account. I had a look and it seems our Support team recently opened a bug for these “authentication failed: Template string must be a string” errors and added you to that. We can’t provide a timeline on when it will be sorted but we’ll be sure to notify you by email as soon as it is. 

If there’s anything else we can help with in the meantime just let us know!
 

And if anyone else comes across that error too, let us know in the comments below and we’ll get you added to the list of users being affected so you’ll also get a notification once it’s fixed.


SamB
Community Manager
Forum|alt.badge.img+11
  • Community Manager
  • Answer
  • November 13, 2025

Hi ​@StuJ, I’m pleased to share that the bug has been marked as fixed! 🎉

Appears to have been an issue with a recent Platform upgrade so the team have reverted the changes associated with the update, which should have resolved it.

Can you try adding your MailerLite account again and confirm whether it works now?