timing loop detected
It could be a bug in your design, if you didn't intend to create a combinatorial feedback loop. Does the warning message tell you which signal is looping?
I found these words somewhere:
"Combinatorial timing loops: These loops are created when output of combinatorial logic or gate is fed back to its input making a timing loop. This kind of loops unnecessary increase the number of cycles by infinitely going around the circle in the same path. These loops also cause a problem in testability."