Thanks for contacting WPML forums support. I'll be glad to help you today.
First of all, sorry for the late reply; it was due to a higher workload.
1) Please log in to this hidden link">sandbox site and replicate the issue. If it is reproducible, please let me know how to reproduce it so I can check it and consult our second-tier support team.
2) Could you please share your Debug information with me?
You can read a detailed explanation about it here. (http://wpml.org/faq/provide-debug-information-faster-support)
The debug info will give me much information about how your site is configured.
Thanks for your reply. I was able to replicate the issue on my local server. I noticed that this warning doesn't appear when you visit (wp-admin/edit-tags.php?taxonomy=post_tag) but appears when you visit (wp-admin/edit-tags.php).
However, I consulted our 2nd tier support team, and I'll update you as soon as they reply.