You would have to mention that you upload around 2500 pictures each time.
You currently have 570 entries in the error_log with the following value:
[CODE title="error_log"][Sun Dec 22 19:36:33.712156 2019] [proxy_fcgi:error] [pid 14770:tid 140566511720192] [client 144.91.xxx.xxx:42324] AH01071: Got error 'PHP message: Request denied'[/CODE]
I myself do not know whether the script is designed for it or whether it is due to the server (php 7.3.13, apache) itself.