Fetch sequences dialog unusable on Linux and gives GUI implementation error warning

On first opening Jalview, with the example project loading as expected, the Fetch Sequences dialog shows a text box with almost no height, and so is effectively unusable.

The Java console shows the following:
Jalview Version: 2.10.1
Jalview Installation: webstart git-commit:260f6ab [releases/Release_2_10_1_Branch]
Build Date: 30 November 2016
Java version: 1.8.0_91
amd64 Linux 4.8.0-27-generic
WARN - GET request for "http://www.ebi.ac.uk/das-srv/registry/das//sources" resulted in 500 (Internal Server Error); invoking error handler
WARN - Squashed a possible GUI implementation error. If you can recreate this, please look at http://issues.jalview.org/browse/JAL-869

JAL-869 seems to relate to loading errors when loading a Jalview project, which is not the case here.