DEBUG urlib3 POST request failed

[MatiaFazio]

Hello everyone.
After creating an API key from the misp web gui with the administrator account and adding on splunk the instance of my MISP, I tried to use the command

| mispgetevent misp_instance=default_misp eventid=5

but the result was as follows:

[MC306] DEBUG urlib3 POST request failed url=https://[my_MISP_domain_name]/events/restSearch, verify=True, header={'Content-type': 'application/json', 'Authorization': '[my_API_key]', 'Accept': 'application/json', 'host': '[my_MISP_domain_name]'}'body={'eventid': '5', 'returnFormat': 'json', 'withAttachments': False, 'page': 1, 'limit': 1000}

I will preface this by saying that on the misp I created some example events, so I confirm that the event with ID=5 exists.
Would anyone be able to explain why?
I have tried both unflagging the "Check MISP certificate" and "Use a client certificate" items but nothing changes, on the contrary, when the second item is flagged when I perform a search I get the error:

"External search command 'mispgetevent' returned error code 1. Script output = "error_message=Exception at "/opt/splunk/etc/apps/misp42splunk/bin/misp_common.py", line 127 : [MC-PC-E05] client_cert_full_path file at /home/docker-misp/ssl/cert.pem not readable "."

The strange thing is that trying to use command like this from the server where the app for splunk misp42 is installed:
curl --header "Authorization: AUTHKEY"
--header "Accept: application/json"
--header "Content-Type: application/json" https://misminstance.com -k (-k because without it I got an SSL error)

I can get all the information, but on MISP platform I see "never" under "last used" column referring on that specific key. Strange, since I had used it a few seconds earlier with the command line command...

Any advice?