[homoeccentricus]

Is there something obvious that can cause a musty flavor in beer that is not caused by poor sanitation or musty ingredients?

[Whiskers]

Immediate effects of oxidation (non-biological effects) can manifest as 'dull' beer, or a loss of 'brightness.'  I suppose that could come across as musty.  Is that at all like what you are describing?  Take a standard, good beer and blast O2 through it for a few minutes and note the changes. 

[homoeccentricus]

(un)fortunately this is not my beer, just asking for a fellow brewer in the Netherlands. So I can't taste it. He claims it's not a sanitation issue.

[dmtaylor]

Sounds like oxidation.  Causes and effects of oxidation are all over the place.... old malt, old extract, aeration at various points and how it was done, how the fermenters are handled, bulk aging prior to bottling/kegging, air intake during bottling or in the keg, just plain age of the beer itself..... or could also be simulated by over-use of crystal/caramel malts......... could be any number of things.

And I always hate to say it, but it truly could be a sanitation or contamination problem.  It happens even to the best brewers sometimes.  Don't think it's not possible, because it IS.  Or the yeast might not have been as healthy as you thought, and something else got in there.  Happens.

[reverseapachemaster]

I would not be quick to rule out sanitation/contamination problems but if we are ruling out a low level infection then the most probable causes are:

1. Oxidation--at any point in the process;

2. Low level phenolics thrown by yeast, particularly due to unhealthy fermentation conditions;

3. Unpleasant brewing water conditions.

Your friend may have a bright future in brewing biere de garde ahead of him.