All plugins, and all code provided by third parties may be under AGPL Licence, you are right.
But
owner of the licence(SAS ProjeQtOr) may pluplish some parts not in the same licence.
It is the same kind of distribution as for Premium / Open source : you can get the free Open Source product, but the full one (Premium) is not open source any more.
In fact, to understand better, you may consider that the whole projeqtor perimeter (Projeqtor main code + plugins + documentation + ...) is premium, not free, private, owned by SAS ProjeQtOr, and owner decided to publish some part (Projeqtor main code) in open source mode.
Especially the point that is limiting the use of the plugin for only one installation per paid license appears to be weird in that context.
This a only way to build a Business model that will garantee thet the product will be maintained and improved for long.
I understand that "source modification does not include the data, neither the parameters, stored in the DB."
Am I right?
Of cource not !!!
You don't have to publish your data under AGPL licence.
Only scripts that build the database are part of the licence, not its result.
AGPL requires that source code should be available to users, in a SaaS mode, users are customers, so the source code is available to the customers but not to the general public.
In AGPL, source code must be provided to Users in AGPL mode, so that Users may then provide code to the general public.
This is designed so that any improvment done on code from projeqtor may get back to projeqtor code.
As a freelancer, I give support to my customer to conceive, manage and deploy organizational and IT projects.
I plan to add projeQtor to my toolbox kit as a saas.
I may provide the service for a fee, but only to my customers: my platform is not made for a "standalone" service.
Most of my "intelligence" is about setting up the parameters, developing a progressive approach for the project.
It may add some code modification - so far it was the "hidden fields".
I have no problem giving the modification of the projector source code.
What about the use of others API, with cron jobs: is that considered to be a modification of the source code?
What you intend to do seems to fit the licence, as long as you provide the code to your customers, that will possibily reverse it to the community.
The best, in the idea of open source, is that you may reverse code yourself to the community, if it has some "common" interest.
Using external APIs from ProjeQtOr needs to code some parts in projeqor code so is concidered as modification of source code.
Using projeqtor API from external programs is not modification of code (it is only use of code, and possibly modification of data)