The mailing list form sometimes results in an error

[VirginiaDooley]

Issue and Steps to Reproduce

Reproduce: Complete the mailing list form, submit, answer CAPTCHA:
Result: {"message":"That feature is not available."}

@chris48s found one github issue about it, buy dunno if that issue describes the set of circumstances we have knadh/listmonk#1913

https://github.com/knadh/listmonk/blob/181948015366c2111013f7121806217a097587b5/i18n/en.json#L326
if you look through https://github.com/knadh/listmonk/blob/master/cmd/public.go there's a few different places where public.invalidFeature gets used in an error response

Screenshots

Expected

Subscription confirmation screen and confirmation mail in inbox

Actual

{"message":"That feature is not available."}

Additional Details

• MacOS, Chrome (both incognito and non)

[chris48s]

I tried reproducing this with @VirginiaDooley 's details.
The form submitted for me.
This leads us to think that what we are seeing is a browser/platform related issue.