What conditions are cause of "CLOSED" with AT+CIPSEND?

[Good Day]

Dear expert,

Occasionally, when I send data with "AT+CIPSEND" for "AT+CIPSENDBUF", I received below return.

Recv xx bytes
CLOSED

After sending data, it takes 7 sec until receive "CLOSED" return.
I wonder my codes have some timing issues or other problems.
So, I want to check conditions why ESP returns "CLOSED" and avoid these conditions.
Would you share why ESP returns "CLOSED" or how I can check problem conditions?

If I need to check "error code" in this condition, please let me know how I can check it.
I try to input "AT+SYSLOG=1" or "AT+SYSLOG?", ESP only returns "ERROR".

Thanks.

p.s. I used ESP-WROOM-02D and please refer below firmware information.

AT+GMR
AT version:1.7.3.0(Mar 19 2020 18:15:04)
SDK version:3.0.3(8427744)
compile time:Mar 24 2020 16:16:12
Bin version(Wroom 02):3.0.3
OK

[Her Mary]

Maybe you can have a try with the keep alive parameter when establishing a connection.