Line 5 of the generated rmlint.sh:

RMLINT_BINARY='(null)'

When running rmlint.sh -p this produces the following errors:

./rmlint.sh: line 119: (null): command not found
 ^^^^^^ Error: files no longer identical - cancelling.....

Further experimentation:

running rmlint test1/ test2/ produces rmlint.sh with RMLINT_BINARY='(null)'

but running /usr/bin/rmlint test1/ test2/ produces rmlint.sh with RMLINT_BINARY='/usr/bin/rmlint'

(This is 2.6.0 compiled and installed from source according to instructions)