I saved the workflow on a different name using save as and the issue stoped.
However yesterday afternoon it started again and now i see i have some nodes working (but taking really long). Similar nodes execute in less than a second and others are loading for hours. I cannot cancel the nodes in order to save them.
some nodes take less and some node take more time to execute. Depending on data and action you are doing. I assume from the screenshot k-Means node are taking longer due to large number of cluster configured. Think about decreasing it or simply wait until they are finished in order to save your workflow. Anyways you have to wait until they are finished in order for succeeding nodes to finish as well so you can have your results.