Dear lan,
thank you very much for your response. I have many text content and very few photos. I did use shortcodes, but only with the LuckyWP Table of Contents for the contents. And I do not believe the error is caused by shortcodes in my case. Because I have just done a small experiment. I have creatived a new post with only a few words. And I did not seen any error massages by saving or updating. Than I started to add more text in the post. Each paragraph only one word. If the number of paragraphs increased to about 10, the error massage started to appear. If I delete a few paragraphs, the error massage will disappear again.