If you have spam problem I can recommend you to read this topic about spam it may be helpful for you:
https://wpforo.com/community/faq/how-to-stop-spam/

I deactivated the plugins and the problem was this plugin Restrict Usernames Emails Characters.

I was using it to prevent the spambots who are all the time registering fake accounts in my forum to put their Advertising in the signature and Account info. It seems to be not copatible with wp foro.

We have checked your website with ours and find out that the problem is not from wpForo, I think that the error comes from an WordPress user plugin, I suggest you to disable that kind of plugins and retry registration.

If the problem won't be from another plugin, we will need administrator credentials that we could find the error, send it by this mail: infogvectors.com

today I noticed that in my forum the user registration doesn´t work anymore. If I want to register a new account I get the message "Error, enter a username". But there is a username given in the registration form.

I was deactivating the cache plugin but it didn´t help. There was a time when everything worked fine.

Please help.