How to debug XWin starting only sometimes?

[Leon Meier]

When starting "XWin Server" though the windows shortcut, it sometimes 
won't start; other times it will.

The shortcut command line is

C:\cygwin64\bin\run.exe --quote /usr/bin/bash.exe -l -c "cd; exec 
/usr/bin/startxwin"

which is executed in directory cygwin64.

The current contents of .xsession-errors is

[
** Message: couldn't connect to dbus session bus: Verbindung ist geschlossen
** Message: couldn't connect to dbus session bus: Verbindung ist geschlossen
** Message: couldn't connect to dbus session bus: Verbindung ist geschlossen
Socket /tmp/fam-admin/fam- has wrong permissions
]

As you can imagine, the directory /tmp/fam-admin contains no files at all.

Starting startxwin from mintty actually succeeds (always or at least 
more often than starting through the shortcut).

The only suspicious lines found in /var/log/xwin/XWin.0.log are

[  1106.609] pixelFormat 36: has 1 overlay, 0 underlays which aren't 
currently handledgetAttrValue failed to find attr 0x2039, using default 
value 0
[  1106.609] getAttrValue failed to find attr 0x203b, using default value 0
...
[  1106.609] pixelFormat 71: has 1 overlay, 0 underlays which aren't 
currently handledgetAttrValue failed to find attr 0x2039, using default 
value 0
[  1106.609] getAttrValue failed to find attr 0x203b, using default value 0
...
[  1294,453] SetupSysMenu: GetSystemMenu() failed for HWND 0x2027a
[  1309,640] IsOverrideRedirect: Failed to get window attributes
[  1311,687] OS has icon alpha channel support: yes
[  1353,109] SetupSysMenu: GetSystemMenu() failed for HWND 0x50374
[  1359,250] SetupSysMenu: GetSystemMenu() failed for HWND 0x50330
[  1367,859] winTopLevelWindowProc - WM_DESTROY - WM_WM_KILL
[  1378,000] SetupSysMenu: GetSystemMenu() failed for HWND 0x1c0466


In /var/log/xwin/XWin.0.log.old we see

[   775,375] winMultiWindowXMsgProc - Fatal error 1 on xcb connection

In /var/log/xwin/XWin.1.log we see

[  1523,000] winMultiWindowXMsgProc - Fatal error 1 on xcb connection

In /var/log/xwin/XWin.1.log.old we see

[  1583,187] winMultiWindowXMsgProcErrorHandler - ERROR: BadMatch 
(invalid parameter attributes)

Any hints who may be the culprit and/or how to debug the issue?

Thanks in advance,
Leon

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