I solved my problem by creating a new strapi project, this time using a PostgreSQL database. To do this, do not use the “–quickstart” flag when creating the project. This way, as indicated in the documentation here, you will be able to choose which type of database you want to use.
There seems to be a problem between PaaS systems and SQLite databases, related to this issue.
I think it should be more clearly indicated in the Strapi documentation that it is recommended not to use SQLite databases if you plan to deploy on PaaS systems.