Can you disable broadcasting local messages in to the dynmap web-chat??
Only global messages should be enabled.

What do you mean local messages? Do you have the chat radius set in the essentials' config.yml?

Exactly! Under local messages i mean chat radius!

And one more suggestion:
When chat-radius is enabled, we have double sending messages in the console:

Example:

[17:26:53 INFO]: [L] Hi! It's a example message.

[17:26:53 INFO]: �[m�[0;30;1m[�[0;33;22mPlayer[0;30;1m] �[0;33;1mNotch�[0;33;22m: �[0;37;1mHi! It's a example message.[m

[17:26:58 INFO]: [L] It's a another example message.

[17:26:58 INFO]: �[m�[0;30;1m[�[0;33;22mPlayer[0;30;1m] �[0;33;1mNotch�[0;33;22m: �[0;37;1mIt's a another example message.�[m

Can you fix that?? (Remove from console messages, which prefixed with [L]).

Seems like an issue with Dynmap. Don't see why it would be pulling messages that are only broadcast to nearby players.