Hey @unborN,
Short Story: it is the apiMaxLength parameter in settings.js that fixes this issue. The httpServerOptions does nothing for this issue.
When testing I had increased the apiMaxLength to 10mb but had actually used so many nodes that the flows file was bigger than the new setting. The original size of the flows file ( .../.node-red/flows_.json was bordering on the default 5mb.
Thanks for the direction.
Skipware