intermittent failures with SelectByLocation in an ArcPy addin [closed]

Question

ArcGIS 10.2, Windows 10

this is the line that fails:

arcpy.SelectLayerByLocation_management(WRS_layer, "INTERSECT", gridLayer, "", "NEW_SELECTION")

The WRS_layer is in a file gdb, the gridLayer is in an SDE gdb. Both are polygon feature classes. SDE layers are being edited by a user-initiated edit session, but in this addin button there is no arcpy.da.Editor. The SelectByLocation will work fine for a while, and then starts giving an error message:

Traceback (most recent call last):
File "C:\Users\...\ESRI\Desktop10.2\AssemblyCache\{1E...D8E}\NORDIS_Landsat.py", line 432, in onClick
   arcpy.SelectLayerByLocation_management(WRS_layer, "INTERSECT", progressLayerName, "", "NEW_SELECTION")
File "C:\Program Files (x86)\ArcGIS\Desktop10.2\arcpy\arcpy\management.py", line 6812, in SelectLayerByLocation
    raise e
ExecuteError: ERROR 999999: Error executing function.
General function failure
General function failure [WRS2_Nordis]
Failed to execute (SelectLayerByLocation).

Here is the referenced line in management.py

6806      from arcpy.geoprocessing._base import gp, gp_fixargs
6807    from arcpy.arcobjects.arcobjectconversion import convertArcObjectToPythonObject
6808    try:
6809        retval = convertArcObjectToPythonObject(gp.SelectLayerByLocation_management(*gp_fixargs((in_layer, overlap_type, select_features, search_distance, selection_type), True)))
6810        return retval
6811    except Exception, e:
6812        raise e

Arcmap doesn't crash, but only a restart of ArcMap solves the problem. I don't think this is a code problem, but I hope that someone else has experienced something similar and can suggest a cause and/or solution.

EDIT: as requested, here is the top part of the button's code immed after def onClick(self)

mxd = arcpy.mapping.MapDocument("CURRENT")
# SET SOME VARIABLES
Error = False
WRS_layer          = "WRS2_Nordis"
WRS_fields         = ["PR"]
username           = getpass.getuser().lower()
rastersPresent     = []
rastersMissing     = []

# 0. VERIFY THAT THE REQUIRED LAYERS ARE PRESENT IN THE MXD.
print
layersPresent = []
for lyr in arcpy.mapping.ListLayers(mxd):
    layersPresent.append(lyr.name)
for lyrReqd in (WRS_layer, progressLayerName):
    if not lyrReqd in layersPresent:
        Error = True
        print "ERROR: cannot find layer  ", lyrReqd, "  in the TOC."

# 1. GET THE SELECTED GRID CELL
d = arcpy.Describe(progressLayerName)
if len(d.FIDSet) == 0:
    ProgressGrid_Select = 0
else:
    ProgressGrid_Select = len(d.FIDSet.split(";"))
if ProgressGrid_Select <> 1:
    print " You have", str(ProgressGrid_Select), " ProgressGrid cells selected  -- should be exactly 1."
    Error = True

# 2. GET THE INTERSECTING WRS SCENES
# make a WRS list of lists, with PathRow

if not Error:
    # SelectLayerByLocation HONOURS THE SELECTION SET IN in_layer
    print 'DEBUG: attempting SelectLayerByLocation_management(WRS_layer, "INTERSECT", progressLayerName, "", "NEW_SELECTION")'
    arcpy.SelectLayerByLocation_management(WRS_layer, "INTERSECT", progressLayerName, "", "NEW_SELECTION")
    #  make list of WRS scenes, with PR (data type Long)
    WRS_scenes = []
    # SearchCursor (in_table, field_names, {where_clause}, {spatial_reference}, {explode_to_points}, {sql_clause})
    with arcpy.da.SearchCursor(WRS_layer,WRS_fields) as cursor:
        for row in cursor:
            WRS_scenes.append(row[0])

    if len(WRS_scenes) == 0:
        print "No WRS_scenes intersect with selected Progress Grid cell."
        Error = True
    else:
        print "Found " + str(len(WRS_scenes)) + " WRS scenes for selected progress cell:"
        WRS_scenes.sort()
        prs = ''
        for pr in WRS_scenes:
            prs += ("  " + str(pr))
        print prs
    print