Unable to locate Signs problem
Solution 1:
Making sure the server software is completely up to date is always a good idea and first place to start.
I'm not sure of the problem's cause, but it's most likely server-side lag. It can be reduced by allocating more RAM to the server. Before you do this, try renaming the 'world' folder of the server. Launching it again will generate a new world. This will fix any world corruption problems.