Is there an easy way to export a MS Access Database backend (Tables & relations) into an SQL Server database, so that it then can be used as a backend for a tailored application written in C# using Entity Framework?
The Access Database contains at least 50 tables and the export should not ruin its structure and relations.
As Gabriel indicates using the SSMA is the best solution for upsizing the tables, indexes and relationships in Access. Then I'd suggest working on the Access front end, containing the queries, forms, reports, macros and VBA code so it works with the SQL Server data storage. This won't take very long by comparision to rewriting the app.
Then you may find you don't need to rewrite the application in another environment. This assumes that the tables are properly normalized and clean. Even then it may be simpler to do some cleanup in that respect in Access.
The Microsoft SSMA link in the comment from 2010 only worked when I chose NOT to register. Also, here is an updated link to the MS SSMA executable.
There are Upsizing Wizard for Access to do the migration to MSSQL 2008. Goto [Database Tools] menu and click on [SQL Server] icon, then proceed the migration with the wizard.
