Unable to submit a review, ojs 3.1.1-4

I use OJS 3.1.1-4. Sometimes the reviewer cannot submit the review form. After clicking “submit” nothing happens apart from displaying cursor showing the system is busy, which can take several minutes. After page reload the form content disappears. It does not happen for every review. Also, in php error log there are no entries with “Error” status, only “Warning”, “Notice” and “Deprecated”.
Where can I search for the cause of this malfunction?