After a long time of searching I have not found any error messages or anything indicating database issues. Then I looked at the PHP configuration and your memory_limit is set to 32M which is very low!!!
WordPress requires at least 64M to run correctly, but it's recommended to use at least 96 – 512M depending on how many plugins you have and how much the server is uses.
I've changed the memory limit via ini_set in your wp-config.php file to 96M (on line 25) and the issue seems to be resolved. If it re-appears just increase the memory limit more slowly, up to 512M if needed.Best,
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