You don't have "left-docks-width" or "right-docks-width" anywhere in your sessionrc? The values there will be different from mine.
You can always try closing GIMP, copying the sessionrc to another location (backing it up) then deleting the sessionrc file in your gimp-2.8 folder. When you restart GIMP, it will create a new sessionrc file with the default values. You should give that a try.
Here is the full contents of my sessionrc file..
# GIMP sessionrc
#
# This file takes session-specific info (that is info, you want to keep
# between two GIMP sessions). You are not supposed to edit it manually, but
# of course you can do. The sessionrc will be entirely rewritten every time
# you quit GIMP. If this file isn't found, defaults are used.
(session-info "toplevel"
(factory-entry "gimp-empty-image-window")
(position 0 32)
(size 1432 831))
(session-info "toplevel"
(factory-entry "gimp-single-image-window")
(position 0 32)
(size 1440 834)
(open-on-exit)
(aux-info
(left-docks-width "247")
(right-docks-width "22")
(maximized "yes"))
(gimp-toolbox
(side left)
(book
(current-page 0)
(dockable "gimp-layer-list"
(tab-style automatic)
(preview-size 48)
(aux-info
(show-button-bar "true")))
(dockable "gimp-tool-options"
(locked)
(tab-style automatic)
(aux-info
(show-button-bar "true")))
(dockable "gimp-font-list"
(tab-style automatic)
(preview-size 48)
(aux-info
(show-button-bar "true")))
(dockable "gimp-pattern-list"
(tab-style automatic)
(preview-size 64)
(aux-info
(show-button-bar "true")))
(dockable "gimp-gradient-list"
(tab-style automatic)
(preview-size 48)
(aux-info
(show-button-bar "true")))
(dockable "gimp-channel-list"
(tab-style automatic)
(preview-size 32)
(aux-info
(show-button-bar "true"))))))
(session-info "toplevel"
(factory-entry "gimp-preferences-dialog")
(position 0 54))
(session-info "toplevel"
(factory-entry "gimp-file-export-dialog")
(position 0 32)
(size 1432 831))
(session-info "toplevel"
(factory-entry "gimp-image-new-dialog")
(position 0 32))
(session-info "toplevel"
(factory-entry "gimp-brightness-contrast-tool-dialog")
(position 0 0))
(session-info "toplevel"
(factory-entry "gimp-file-save-dialog")
(position 0 32)
(size 1432 831))
(session-info "toplevel"
(factory-entry "gimp-file-open-dialog")
(position 0 32)
(size 1432 831))
(session-info "toplevel"
(factory-entry "gimp-toolbox-color-dialog")
(position 0 0))
(session-info "toplevel"
(factory-entry "gimp-hue-saturation-tool-dialog")
(position 0 0))
(session-info "toplevel"
(factory-entry "gimp-color-balance-tool-dialog")
(position 0 0))
(session-info "toplevel"
(factory-entry "gimp-colorize-tool-dialog")
(position 114 0))
(session-info "toplevel"
(factory-entry "gimp-desaturate-tool-dialog")
(position 191 0))
(session-info "toplevel"
(factory-entry "gimp-levels-tool-dialog")
(position 265 0)
(size 518 641))
(session-info "toplevel"
(factory-entry "gimp-scale-tool-dialog")
(position 326 450))
(session-info "toplevel"
(factory-entry "gimp-file-open-location-dialog")
(position 0 175))
(session-info "toplevel"
(factory-entry "gimp-curves-tool-dialog")
(position 222 0)
(size 518 583))
(session-info "toplevel"
(factory-entry "gimp-perspective-tool-dialog")
(position 411 22))
(session-info "toplevel"
(factory-entry "gimp-shear-tool-dialog")
(position 527 162))
(session-info "toplevel"
(factory-entry "gimp-input-devices-dialog")
(position 97 79))
(hide-docks yes)
(single-window-mode yes)
(last-tip-shown 0)
# end of sessionrc