You mention two issues:
#1. Difficulty in Registering
#2. Inability to login after registering.
Please distinguish between them.
For #1
+ What are the symptoms that your users complain about?
+ Can they access the registration screen "Register" link on the forum?
+ Can they complete the Captcha field without error?
+ Do they received an email with a confirmation link?
+ What do they see on clicking the confirmation link?
For #2
+ Are your users listed as registered when you click the "People" link on your forum?
+ What messages do they see when attempting to login?
You need to be far more detailed in your description of the problems if there is to be any chance of resolving your issues.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.