when SOME people try and add their email to the mailing list they get this error:
The following errors were encountered
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ExpressionEngine uses an anti CSRF technique where it assigns a value to a hidden field (i believe the field name is
XID in version < 2.8.0) and assigns the same value to a session. At form processing stage it compares the two and if they dont match the form returns an error. I believe the error you are getting is the one that gets returned when this happens.
If the visitor is getting this error, the form might be cached at the server / network level, delivering them a stale CSRF token. Are all of the affected users on the same network? Corporate firewalls could be delivering stale pages, for instance.