Since you already have created an account with WeChat, you can simply relogin and then add your friend again. This is available for your friend as well. If there are no blocking measures in place for one of your usernames, this should work normally and what you experienced could be a simple error.
I have searched for this issue, but I couldn't find anything similar.
If nothing helps to resolve the issue, contact the developers of this Android application via email@example.com.