To whom it may concern
Although the threat is meanwhile a bit stale, this bit of information might be somewhat relevant:
I stumbled over a post in a german F800 forum describing occasional stalling after riding in the rain. Although the initial stalling occured during a bad weather run, later failures occured even during rides in pretty good conditions and also pretty unfrequently. After faffing around for quite some time without being able to isolate the problem, the guy did finally get around reading out the error memory: It flagged up several "fuel pump failure" warnings.
This motivated him to have a closer look at the fuel pump / fuel pump electronics: Turned out that although the fuel pump assembly under the seat is protected by a rubber seal (directly under the drivers b...), he found to his surprise under the membrane quite some moisture. Dismantling the assembly even revealed the plug for the fuel pump partially submerged.
Careful drying, subsequent contact cleaning and changing the rubber seal (30$) did the trick for him: No re-occurance.
Don't know if this is only an isolated case, but maybe worth for the people experiencing similar problems to have a look under that rubber membrane...