Been running Icinga2 for a few years now, and love it - so firstly thanks for a great piece of software!
For quite a while now I have been unable to acknowledge warnings via the icingaweb2 UI. I found this bug which looks very similar to what I am seeing, but it says that was resolved in v2.4.2.
I am currently running v2.6.2 of icingaweb2. And the icinga2 daemon version is;
icinga2 - The Icinga 2 network monitoring daemon (version: r2.10.2-1) Copyright (c) 2012-2018 Icinga Development Team (https://icinga.com/) License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl2.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. System information: Platform: Debian GNU/Linux Platform version: 8 (jessie) Kernel: Linux Kernel version: 4.15.18-11-pve Architecture: x86_64