No callback on http request

I re-routed the http request form the Pico trough a proxy on my computer. The request itself is working but the Pico doesn't seem to close the connection. Also sometimes (10% of the time) not all the payload data is sent (it gets stuck at 272 bytes). I added a wifi.at.debug and after the data is sent the following is printed by at.debug:

] "> \r" <--- "\r"
] "\nER" <--- "\nER"
] "ERRO" <--- "RO"
] "ERROR" <--- "R"
] "ERROR\r\n" <--- "\r\n"
] "Unl" <--- "Unl"
] "Unlin" <--- "in"
] "Unlink" <--- "k"
] "Unlink\r\n" <--- "\r\n"
["AT+CIPCLOSE=0\r\n"
] "l" <--- "l"
] "li" <--- "i"
] "link" <--- "nk"
] "link i" <--- " i"
] "link is" <--- "s"
] "link is n" <--- " n"
] "link is not" <--- "ot"
] "link is not\r\n" <--- "\r\n"

I guess this error is the problem. But I don't know where its coming from.