It’s not easy to troubleshoot… The only thing I could find in the Apache log:
Order: new to old
.263163 [NOTICE] [73228] [T0] [x.x.x.x:64117:HTTP2-877#APVH_websitename.nl:443] oops! 503 Service Unavailable
.263167 [NOTICE] [73228] [T0] [x.x.x.x:64117:HTTP2-877#APVH_websitename.nl:443] Content len: 92, Request line: ‘POST /wp-admin/admin-ajax.php HTTP/1.1’
.263169 [INFO] [73228] [T0] [x.x.x.x:64117:HTTP2-877#APVH_websitename.nl:443] Cookie len: 613, wordpress_sec_3a31d67b7b2bbf9120cc80a9f81225e0=adminname%7C1678977715%7CvXx2Xr4RcRIC5swwLrXlvCXlFi8scBTMcQbZPlTL9q1%7Ce1826a2fdb7ed59087361a4516101bc22d5622beae95e02a675f6771029c42a1; wordpress_test_cookie=WP%20Cookie%20check; wordpress_logged_in_3a31d67b7b2bbf9120cc80a9f81225e0=adminname%7C1678977715%7CvXx2Xr4RcRIC5swwLrXlvCXlFi8scBTMcQbZPlTL9q1%7C7f2b6b0bbe04cd9d82359cd1c667690971ecae7d61317eb0b3a82d0a0146f9d5; wp-settings-1=deleted%3Dundefined%26ampimgsize%3D%26amplibraryContent%3Dbrowse%26imgsize%3D%26libraryContent%3Dbrowse%26posts_list_mode%3Dlist%26editor%3Dtinymce; wp-settings-time-1=1678804917
.384300 [INFO] [73228] [T0] [x.x.x.x:64117:HTTP2-875#APVH_websitename.nl:443] connection to [uds://usr/local/lsws/extapp-sock/APVH_gjbljxpj_Suphp80.sock] on request #1, confirmed, 1, associated process: 111336, running: 1, error: Connection reset by peer