Failed to add documents to Solr: Solr responded with an error (HTTP 400) (django + haystack + solr)

I currently have Solr 4.2.0 working in production (set up around 2012). I have set up a new development environment where I upgraded all packages (Django 1.8.10, PySolr 3.4.0, Haystack 2.4.1) and set up Solr 5.5.0

In short

I have Solr running, my core/collection created with 'basic_configs' and it seems to work well, except that during indexing I get a lot of errors similar to these:

All documents removed.
Indexing 9604 contracts
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.22] unknown field 'status']
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.70556] unknown field 'date_signed']
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.72059] unknown field 'date_signed']
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.73458] unknown field 'date_signed']

Looking at the id's, it seems most documents are fine, but frequent enough (the list goes on) these errors appear throughout all tables/indexes.

Eventually I followed this promising github project guide, but unfortunately it did not solve the problems for me.

What I did, step by step

1. Succesfully installed Solr 5.5.0 (web interface working at
   localhost:8983), using this guide
2. Created a collection called 'spng', using the following command: sudo su - solr -c '/opt/solr/bin/solr create -c spng -d basic_configs'
3. Overwritten my solr.xml (/srv/spng/src/django-haystack/haystack/templates/search_configuration/solr.xml) with the solr.xml from the earlier mentioned github project guide
4. Just to be sure I gave the solr.xml file 777 rights.

My settings.py has the following entry:

HAYSTACK_CONNECTIONS = {
    'default': {
        'ENGINE': 'haystack.backends.solr_backend.SolrEngine',
        'URL': 'http://localhost:8983/solr/spng',
        'DEFAULT_OPERATOR': 'AND',
        'INCLUDE_SPELLING': True,
    },
}

5. I created a schema.xml (python manage.py build_solr_schema) and placed it in /var/solr/data/spng/conf/schema.xml
6. Again, just to be sure I gave the schema.xml file also 777 rights.
7. I used the curl command to reload the core: curl 'http://localhost:8983/solr/admin/cores?action=RELOAD&core=spng&wt=json&indent=true'

The response was:

{
  "responseHeader":{
    "status":0,
    "QTime":300}}

8. I also restarted uwsgi and solr just to make sure
9. At this point I try to run the python manage.py rebuild_index command

I end up with the following errors, as mentioned before:

All documents removed.
Indexing 9604 contracts
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.22] unknown field 'status']
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.70556] unknown field 'date_signed']
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.72059] unknown field 'date_signed']
Failed to add documents to Solr: Solr responded with an error (HTTP 400): [Reason: ERROR: [d
oc=accounting.contract.73458] unknown field 'date_signed']

Does anyone have any idea what might be wrong? The indexing works without errors on my production server, running 4.2.0. Did I miss a setting or is Solr 5.5.0 causing these errors?