There's currently a known issue affecting invitation links from friends in Version 5.2.2. If you try to use the link without having Monster Strike running in the background, an error will occur and you won't be able to become friends.
If this error occurs, try tapping the invitation link again with Monster Strike running in the background.
- If you're the inviter and notice your friend having trouble with this issue, share the knowledge!
This issue is currently slated to be fixed in the next version.
Thank you for your patience!
The issue has been fixed. Thanks for your patience!