As I said in my reply to your PM, if you have any fields on your join form that are not mandatory, make sure the minimum required characters for those fields is set to zero.
I don't know if it's fixed, but in a previous version, a field could be optional (Not mandatory), but the form would not validate if the field was left blank and the minimum number of characters was 1 or more. I just changed the minimum number of characters to zero for fields like the description, which I want to be optional. If you force people to fill out this field, they'll just type a bunch of random characters in it half the time.
Others might not agree, but if the field is not marked as mandatory, the minimum character test should be bypassed.
I don't know if this is your problem, but it's something to check.
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