I installed Craft CMS 3.0.1 locally on MAMP (Apache/PHP7.0.22), but when I want to do a backup of the database it says: "There was a problem backing up your database. Please check the Craft logs. Back-up van database". I cound not find any logs that pin pointed the problem. How does this happen? Thanks in advance.
I have just run into a similar problem.
I'm not sure if this is what fixed it but try creating an Envvars file as per jonlivingstone's comment here https://github.com/craftcms/cms/issues/2364
Once I had done that my backup ran. I then had to switch MAMP out of FastCGI mode otherwise I got a 500 error at the end of the update process.