I found the solution. The problem has to do with the disk usage in total as described in the answer from sastorsl here:
I was working on a cluster the storage of which was 98% used, still there were 400GB free, but Elasticsearch only looks at the percentages, thus shutting down any write permissions of indices.
The solution is to manually set the watermarks after the nodes have started (setting them in the elasticsearch.yml didn't work for some reason):
curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_cluster/settings -d '{ "transient": { "cluster.routing.allocation.disk.threshold_enabled": false } }'
curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_all/_settings -d '{"index.blocks.read_only_allow_delete": null}'
Of course you have to put in your index names.
After that, they will be writable again.