It turns out that an empty QComboBox evaluates to False:
>>> t=QLineEdit()
>>> bool(t)
True
>>> c=QComboBox()
>>> bool(c)
False
Due to that, the 'TABLE' parameter was missing if its (editable) combo box was empty.
Original error - while updating an algorithm's parameters (gdal:importvectorintopostgisdatabaseavailableconnections)
Traceback (most recent call last):
File "/home/martin/qgis/git-master/build-debug/output/python/plugins/processing/algs/gdal/GdalAlgorithmDialog.py", line 121, in parametersHaveChanged
or (not p.checkValueIsAcceptable(parameters[p.name()])):
KeyError: 'TABLE'
80b0c72c50