Sometimes a wrong XML answer is generated. Can't really reproduce it. All those misformatted answers look like:

<?xml version="1.0" encoding="utf-8"?><!DOCTYPE ActiveSync PUBLIC "-//MICROSOFT//DTD ActiveSync//EN" "http://www.microsoft.com/&quot;>&lt;Sync xmlns="AirSync:"><Collections><Status>13</Status></Sync>

(Missing </Collections> tag)

In logs I can see something like:
Jan 08 10:36:45 sogod [19327]: <0x0x560d8272e200[SOGoActiveSyncDispatcher]> EAS - Discard response {FolderSyncKey = "activesync_company_com_1942_23ab6a7e_0"; PingCachedFolders = ("mail%2Fee293c324575615baa51000048ac2651", "vevent%2Fpersonal", "vcard%2Fpersonal", "mail%2F90a38300157b1b5ced1a000048ac2651", "mail%2F71b26138cfad625bd975000048ac2651", "vtodo%2Fpersonal", "mail%2F19c1910991673c5cbd57000048ac2651", "mail%2F727273071f911b5c3c12000048ac2651", "mail%2FJunk", "mail%2Fb5ff021736673c5c6557000048ac2651", "mail%2Fshared", "vevent%2Fstaff-calendar_personal", "vtodo%2Fstaff-calendar_personal"); PingHeartbeatInterval = 354; PingRequest = 19331; SyncRequest = 1578476202; "SyncRequest+mail/19c1910991673c5cbd57000048ac2651" = 19331; "SyncRequest+mail/71b26138cfad625bd975000048ac2651" = 19331; "SyncRequest+mail/727273071f911b5c3c12000048ac2651" = 19331; "SyncRequest+mail/90a38300157b1b5ced1a000048ac2651" = 19331; "SyncRequest+mail/Junk" = 19331; "SyncRequest+mail/b5ff021736673c5c6557000048ac2651" = 19331; "SyncRequest+mail/ee293c324575615baa51000048ac2651" = 19331; "SyncRequest+mail/shared" = 19331; "SyncRequest+vcard/personal" = 19331; "SyncRequest+vevent/personal" = 19331; "SyncRequest+vevent/user_personal" = 19331; "SyncRequest+vtodo/personal" = 19331; "SyncRequest+vtodo/user_personal" = 19331; }
Jan 08 10:36:45 sogod [19327]: [ERROR] <0x0x560d80708020[NSDataMalloc]> xml2wbxmlFromContent: failed: Parsing of XML Document Failed

Jan 08 10:36:45 sogod [19327]: [ERROR] <0x0x560d80708020[NSDataMalloc]> Original data written to: /tmp/activesync_benocs_com_4b7f_23c51684_1.data