Just refreshed org from the package manager.
org 20190415 installed Outline-based notes management and organizer
org-plus-contrib 20190415 installed Outline-based notes management and organizer
org-drill-entry: Symbol’s function definition is void: org-outline-overlay-data
I was able to fix the problem by grepping for references in *.elc files and byte-compiling the files.
sloth% ls -lrt | tail -8
-rw-r--r-- 1 dap staff 12041 Apr 15 21:36 ox-rss.elc
-rw-r--r-- 1 dap staff 13775 Apr 15 21:36 ox-s5.elc
-rw-r--r-- 1 dap staff 29636 Apr 15 21:36 ox-taskjuggler.elc
-rw-r--r-- 1 dap staff 58283 Apr 15 21:36 ox-texinfo.elc
-rw-r--r-- 1 dap staff 242279 Apr 15 21:36 ox.elc
-rw-r--r-- 1 dap staff 100111 Apr 18 06:10 org-drill.elc
-rw-r--r-- 1 dap staff 102041 Apr 18 06:11 ob-core.elc
-rw-r--r-- 1 dap staff 796772 Apr 18 06:11 org.elc
sloth% pwd
/Users/dap/.emacs.d/elpa/org-plus-contrib-20190415
sloth%