It does sound like you have checked most things. Make sure that your user roles have the capability
1. I would suggest that you only enable one coin, such as Litecoin testnet, then wait two minutes and try again. It is possible that some coin that you have enabled triggers an error on the API, and if too many errors occur, the API bans you for two minutes. IF this is the case, then there should be some information in the WordPress logs, once you enable them.
2. Make sure that the
wp_wallets_adds table exists. If it does not, the plugin will warn you about this. In the admin screens there will be an error message at the top of the screen.
Let me know if this helps.