There is an error on edit module page for lesson (not present on add module page):

( ! ) Notice: Undefined property: stdClass::$version in /home/marina/repositories/master/moodle/mod/lesson/mod_form.php on line 321

Call Stack

#	Time	Memory	Function	Location

1	0.0011	316480	{main}( )	../modedit.php:0

2	0.4141	33024360	moodleform_mod->set_data( )	../modedit.php:256

3	0.4141	33031072	mod_lesson_mod_form->data_preprocessing( )	../moodleform_mod.php:416

I don't really have time to trace what caused the regression and why global $module object does not exist any more in 2.6.
This is some old code that checks the version, which is obsolete for last 5 years because version number can't start with 2008. Can be easily removed and maybe from 2.4 and 2.5 as well even though there are no warnings there