org-agenda not showing any todos

No items are showing when I load the agenda view. When I press C-c a, the agenda view menu comes up fine. But hitting a brings up an empty list, and so does t.

M-x describe-variable RET org-agenda-files shows

There are tonnes of TODO items in ~/org/todo.org, eg:

My init is defined in ~/.emacs.d/init.el and I do not have an ~/.emacs. My init.el has only one definition of org-agenda-files.

I really am at a loss as to why this is not working.

Emacs version 26.3
Org version 9.3.1

Not sure what other information might be useful?

I have been using Emacs for a while, but would consider myself a noob with custom configuration and org mode.

UPDATE:

The Messages buffer is showing errors like this:

Press key for agenda command (unrestricted):
org-agenda-get-todos: Wrong type argument: number-or-marker-p, "E"
user-error: Command not allowed in this line [3 times] 

I set debug-on-entry for org-agenda-get-todos, and it shows this in the debug buffer:

Debugger entered--entering a function:
* org-agenda-get-todos()
org-agenda-get-day-entries("~/org/todo.org" (1 21 2020) :todo)
org-todo-list(nil)
funcall-interactively(org-todo-list nil)
call-interactively(org-todo-list)
org-agenda(nil)
funcall-interactively(org-agenda nil)
call-interactively(org-agenda nil nil)
command-execute(org-agenda) 

(setq org-highest-priority "A")
(setq org-lowest-priority "E")
(setq org-default-priority "B")

(setq org-highest-priority ?A)
(setq org-lowest-priority ?E)
(setq org-default-priority ?B)