V 4.2.1 all of the sudden my addons just disabled automatically… any idea why?
Usually it is either a fatal php error during initialization and/or a database error. But it's usually the former.
You can check your PHP error_log file and search for 'addons/' which should take you to the errors (note that Notice or Warnings will not cause them to disable, only Fatal).
Please also note that some add-ons can be disabled automatically when you activate an incompatible add-on.
Really? So a running addon that a merchant has been using can be disabled when the merchant installs a new addon they've never used before but the author has deemed the other addon as incompatible? So competing addon developers can just identify other developer's addons as incompatible?
If this is true, this is a really bad idea.
In the documentation we can find the following:
Conflicts. These add-ons will be automatically disabled before the current add-on installation starts and the notification will be displayed
That's just plain WRONG. It is the addon being installed that should be disabled with a notice of what it's incompatible and also a log entry about it so someone can do a postmortem.
Should never allow one addon to disable another. That would be fine if all addons came from one place, but they don't.