Thank you for sharing your situation with the Wefugees Community and happy new year 2020!
As most of the European countries, Germany and Belgium apply the so-called Dublin regulation. The Belgian authorities will most likely refer to your rejected case in Germany and not accept your new application.
I am not an expert on requirements for getting married in Belgium, but I am quite sure that they will ask for documents as a valid passport, birth certificate, document proving your civil status etc.
However, if you are able to provide all those documents, you may also be able to marry in Germany, as it is in general not dependent on the residence status. You may want to read our blog on this topic. It lists all the required documents also.
Please feel free to get back to us with any further questions.
All the best,