PCB solder mask inconsistency
We had a detailed discussion with the PCB manufacturer. Suffice to say, that they guarantee (according to their "Process and Capability Manual") a solder mask of at least 5um. 

According to them, everything nearing the 5um thickness would already result in the "copperish" color shining through the solder resist.

The solder resists viscosity may vary slightly and cause differing results, especially at the edges, where some resist may flow off the copper.

Nevertheless we get better and more consistent results from other manufacturers, so it still stands that their processing quality may not be optimal.