Xpra: Ticket #2548: local xpra attach warning

I received the following warning while trying to locally attach an xpra session. Along with an error dialog I didn't know what to do about.

$ xpra attach 20
xpra for python 2.7 is not installed
 retrying with python3
2020-01-13 14:41:21,037 Xpra GTK3 X11 client version 3.0.5-r24939 64-bit
2020-01-13 14:41:21,112  running on Linux Ubuntu 16.04 xenial
2020-01-13 14:41:21,113  window manager is 'Compiz'
2020-01-13 14:41:21,257 No OpenGL_accelerate module loaded: No module named 'OpenGL_accelerate'
2020-01-13 14:41:21,531 OpenGL enabled with Quadro P400/PCIe/SSE2
2020-01-13 14:41:21,558  keyboard settings: layout=us, rules=evdev, model=pc105
2020-01-13 14:41:21,562  desktop size is 6400x1440 with 1 screen:
2020-01-13 14:41:21,563   :0.0 (1693x381 mm - DPI: 96x96) workarea: 6341x1416 at 59x24
2020-01-13 14:41:21,563     DP-0 2560x1440 (597x336 mm - DPI: 108x108)
2020-01-13 14:41:21,563     DP-2 1920x1080 at 2560x180 (527x296 mm - DPI: 92x92)
2020-01-13 14:41:21,563     DP-4 1920x1080 at 4480x180 (527x296 mm - DPI: 92x92)
2020-01-13 14:41:21,563  upscaled to 125%, virtual screen size: 5120x1152
2020-01-13 14:41:21,563   :0.0 (1693x381 mm - DPI: 76x76) workarea: 5073x1133 at 47x19
2020-01-13 14:41:21,563     DP-0 2048x1152 (597x336 mm - DPI: 87x87)
2020-01-13 14:41:21,563     DP-2 1536x864 at 2048x144 (527x296 mm - DPI: 74x74)
2020-01-13 14:41:21,563     DP-4 1536x864 at 3584x144 (527x296 mm - DPI: 74x74)
2020-01-13 14:41:21,599 Warning: invalid frame extents value '[0, 0, 0, 0, 0, 0, 28, 0]'
2020-01-13 14:41:21,600  this is probably a bug in 'Compiz'
2020-01-13 14:41:21,600  using '[0, 0, 28, 0]' instead
2020-01-13 14:41:22,214 enabled fast mmap transfers using 281MB shared memory area
2020-01-13 14:41:22,214 enabled remote logging
2020-01-13 14:41:22,215 Xpra GTK3 X11 server version 3.0.4-r24778 64-bit
2020-01-13 14:41:22,215  running on Linux Ubuntu 16.04 xenial
2020-01-13 14:41:22,216  no need for scaling with mmap
2020-01-13 14:41:22,218 Warning: Invalid Scale Factor
2020-01-13 14:41:22,218  cannot scale by 100% x 100% or lower
2020-01-13 14:41:22,218  the scaled client screen 6400 x 1440 -> 6400 x 1440
2020-01-13 14:41:22,218   would overflow the server's screen: 5760 x 2560
2020-01-13 14:41:22,223 Attached to socket:///run/user/1000/xpra/user-20
2020-01-13 14:41:22,223  (press Control-C to detach)
2020-01-13 14:41:22,543 server does not support xi input devices
2020-01-13 14:41:22,543  server uses: xtest

The session was normaly used on Win10 laptop (1366x768, see #2527), but now it was attached normally for cleanup.

I also don't know what I was supposed to do with the error dialog that appeared: Red X, 5 buttons: 'c', 'i', '*', 'cancel?', 'ok' - and a similar warning message