Same problem here: we’ve got four iphones 6 & 7. One have updated to ios 12, and then had no more reacted to authentication “status 401” from nextcloud.
Other iphones (until "iOS/11.4.1 (15G77) dataaccessd/1.0"… ) and thunderbird/Lightning (on windows) are still working well; the fault seems to come from ios service “iOS/12.0 (16A366) dataaccessd/1.0”.
FreeBSD 11 + nginx 14 + php7.1 + postgresql 9.6 + nextcloud 14 (all upgraded and uptodate as yesterday).
nextcloud: http (not ssl) in lan only.
Any help would be great…