2 SG550, webproxy stop working, unable to parse

WWe have 2 SG550 (9.707-5) which recently have problems with the web proxy.  Currently, web surfing is no longer possible when the web proxy is enabled. For the users (~600), the browser then only spins in the blank, the searched website is not displayed. As soon as the webproxy is deactivated, everything runs again immediately.
The webproxy runs in transparent mode.
We have already deactivated https scanning for test purposes. We have also deactivated the virus scan.

Here is a log of the web proxy, when the problem occurs

2022:01:12-14:22:09 firewall-1 httpproxy[27600]: id="0003" severity="info" sys="SecureWeb" sub="http" request="(nil)" function="http_parser_context_execute" file="http_parser_context.c" line="97" message="Unable to parse a http message of 96 bytes (HPE_INVALID_METHOD: invalid HTTP method)"
2022:01:12-14:22:09 firewall-1 httpproxy[27600]: id="0003" severity="info" sys="SecureWeb" sub="http" request="0x32a37500" function="read_request_headers" file="request.c" line="1623" message="unable to parse a http message on handler 8508 (Resource temporarily unavailable)"
2022:01:12-14:22:09 firewall-1 httpproxy[27600]: id="0003" severity="info" sys="SecureWeb" sub="http" request="(nil)" function="confd_config_reload_func" file="confd-client.c" line="594" message="reloading config"

We are stumped and really stuck here.
Thanks in advance for your inputs!