Likely you are talking about this ucert error:
- Latest openwrt git cant build (08.08.2018, ucert - libjson-c.so)
- https://github.com/openwrt/openwrt/pull/1259
(includes a patch) - https://bugs.openwrt.org/index.php?do=details&task_id=1754&order=id&sort=desc
- https://www.mail-archive.com/openwrt-devel@lists.openwrt.org/msg43032.html
I used that patch once to create the new ucert build keys, and noticed that after that you do not even need the patch, if you just copy the new keys all your possible buildroots (I have 3 parallel master buildroots).
Alternatively, you can apparently also disable packages list signing, but I have not tried that.