I'm not sure exactly what happens here, but it looks like the linker is handed a nameless file. No problems with the OMF build.

c:\prog\prosjekt\MonsterBrowser_phobos2>dub build -a=x86_mscoff -v
Using dub registry url 'https://code.dlang.org/'
Refreshing local packages (refresh existing: true)...
Looking for local package map at C:\ProgramData\dub\packages\local-packages.json
Looking for local package map at C:\Users\Tor\AppData\Local\dub\packages\local-packages.json
Try to load local package map at C:\Users\Tor\AppData\Local\dub\packages\local-packages.json
Looking for local package map at c:\prog\prosjekt\MonsterBrowser_phobos2\.dub\packages\local-packages.json
Determined package version using GIT: monster-browser ~phobos2
Refreshing local packages (refresh existing: false)...
Looking for local package map at C:\ProgramData\dub\packages\local-packages.json
Looking for local package map at C:\Users\Tor\AppData\Local\dub\packages\local-packages.json
Try to load local package map at C:\Users\Tor\AppData\Local\dub\packages\local-packages.json
Looking for local package map at c:\prog\prosjekt\MonsterBrowser_phobos2\.dub\packages\local-packages.json
  Found dependency dwt 1.0.5+swt-3.4.1
    Found dependency dwt:base 1.0.5+swt-3.4.1
Refreshing local packages (refresh existing: false)...
Looking for local package map at C:\ProgramData\dub\packages\local-packages.json
Looking for local package map at C:\Users\Tor\AppData\Local\dub\packages\local-packages.json
Try to load local package map at C:\Users\Tor\AppData\Local\dub\packages\local-packages.json
Looking for local package map at c:\prog\prosjekt\MonsterBrowser_phobos2\.dub\packages\local-packages.json
  Found dependency dwt 1.0.5+swt-3.4.1
    Found dependency dwt:base 1.0.5+swt-3.4.1
Generating using build
Configuring dependent monster-browser, deps:"dwt"
  Configuring dependent dwt, deps:"dwt:base"
    Configuring dependent dwt:base, deps:
Performing "debug" build using dmd for x86, x86_mscoff.
dwt:base 1.0.5+swt-3.4.1: target for configuration "library" is up to date.
Using existing build in C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\.dub\build\library-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-0484A19E0EA2898152475339D8FB31F2\.
Copying target from C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\.dub\build\library-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-0484A19E0EA2898152475339D8FB31F2\dwt_base.lib to C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt
dwt 1.0.5+swt-3.4.1: target for configuration "windows-win32" is up to date.
Using existing build in C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\.dub\build\windows-win32-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-C4D8C66136FA49EF266284422BAE7F31\.
Copying target from C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\.dub\build\windows-win32-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-C4D8C66136FA49EF266284422BAE7F31\dwt.lib to C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt
Target 'c:\prog\prosjekt\MonsterBrowser_phobos2\.dub\build\application-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-62D1E946E174AAA0E39CE7B8954ED116\MonsterBrowser.exe' doesn't exist, need rebuild.
monster-browser ~phobos2: building configuration "application"...
dmd -m32mscoff -c -of.dub\build\application-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-62D1E946E174AAA0E39CE7B8954ED116\MonsterBrowser.obj -debug -g -d -version=Have_monster_browser -version=Have_dwt -version=Have_dwt_base -Isrc -IC:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\org.eclipse.swt.win32.win32.x86\src -IC:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\base\src -J. -Jflags -Jicons -Jres -JC:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\org.eclipse.swt.win32.win32.x86\res src\colorednames.d src\common.d src\cvartable.d src\dialogs.d src\flagdata.d src\geoip.d src\ini.d src\launch.d src\lib\loader.d src\lib\phobosfixes.d src\lib\pipestream.d src\lib\process.d src\link.d src\main.d src\mainwindow.d src\masterlist.d src\messageboxes.d src\mswindows\taskbarlist.d src\mswindows\taskbarprogress.d src\mswindows\util.d src\playertable.d src\qstat.d src\rcon.d src\runtools.d src\serveractions.d src\serverdata.d src\serverlist.d src\serverqueue.d src\servertable.d src\set.d src\settings.d src\tango\core\Array.d src\tango\core\Octal.d src\tango\core\Traits.d src\tango\sys\win32\CodePage.d src\tango\sys\win32\IUnknown.d src\tango\sys\win32\Macros.d src\tango\sys\win32\SpecialPath.d src\tango\sys\win32\Types.d src\tango\sys\win32\UserGdi.d src\tango\text\Ascii.d src\tango\text\Util.d src\tango\text\convert\Integer.d src\tango\text\xml\PullParser.d src\tango\text\xml\SaxParser.d src\threadmanager.d src\undead\doformat.d src\undead\internal\file.d src\undead\stream.d src\undead\utf.d -vcolumns
Linking...
dmd -of.dub\build\application-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-62D1E946E174AAA0E39CE7B8954ED116\MonsterBrowser.exe .dub\build\application-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-62D1E946E174AAA0E39CE7B8954ED116\MonsterBrowser.obj C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\.dub\build\windows-win32-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-C4D8C66136FA49EF266284422BAE7F31\dwt.lib C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\.dub\build\library-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-0484A19E0EA2898152475339D8FB31F2\dwt_base.lib advapi32.lib comctl32.lib comdlg32.lib gdi32.lib kernel32.lib msimg32.lib ole32.lib oleacc.lib oleaut32.lib opengl32.lib shell32.lib shlwapi.lib user32.lib usp10.lib olepro32.lib -L/subsystem:console,5.01 -L/rc:misc\dwt.res -L/rc:misc\mb.res -L+C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\org.eclipse.swt.win32.win32.x86\lib\ -L/exet:nt/su:console:4.0 -m32mscoff -g
LINK : warning LNK4044: unrecognized option '/exet:nt/su:console:4.0'; ignored
LINK : fatal error LNK1104: cannot open file '+C:\Users\Tor\AppData\Local\dub\packages\dwt-1.0.5_swt-3.4.1\dwt\org.eclipse.swt.win32.win32.x86\lib\.obj'
Error: linker exited with status 1104
FAIL .dub\build\application-debug-windows-x86.x86_mscoff-dmd_v2.094.2-dirty-62D1E946E174AAA0E39CE7B8954ED116\ MonsterBrowser executable
dmd failed with exit code 1.