After installing org-ql-20191207.408 via use-package (yes, I switched from GitHub version to use-package because of list of dependencies) I get this when invoking helm-org-ql:

Error running timer: (error "In ‘helm-org-ql’ source: ‘#[0 \\303	!\\304\\305 !�\\205%\0r\\306 q\\210\\300�)\\3071#\0\\310\\300�\\311\\312�D$0\\202%\0\\210\\313\\207 [issues.org helm-pattern helm-org-ql-buffers-files org-ql--plain-query window-width helm-window helm-buffer-get (error) org-ql-select :action helm-org-ql--heading nil] 8]’ 
 (void-function peg-parse-string)")

Initial tests of org-ql-search do seem to work though.

Some package versions I do think might be involved:

• helm-org-20191214.802
• org-ql-20191207.408
• ts-20191010.210
• peg-1.0
• ov-20191127.32
• dash-functional-20191109.1327
• org-sidebar-20191012.514