Hi!
Sounds like there is confusion with the Apps in the SEBLOD store vs the Apps that you create and then download to install on another site.
You do not need a subscription for downloading your own Apps.
Is the problem on a local or hosted site?
I have had this same issue on my local setups. But the App.Zip file still gets created and can be found the tmp folder so I just ignore the message. If this is not on a local machine, then it usually is because a user rights issue.
You say that it was all of a sudden. So there must have been an update of some kind? If it wasn't you, then perhaps the host?
If its on a server, then did the host make an update and change the PHP settings?
What version of Joomla/Seblod/PHP?