ResMerger errFSBadFSRef when compiling Photoshop plug-in

I am trying to build WebPShop, a Photoshop plug-in. It compiles fine on Mac OS X 10.14.6 18G3020 with xcodebuild 11.3 11C29. Here is the log of the working Rez step:

ResMergerCollector /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/WebPShop.rsrc (in target 'WebPShop' from project 'webpshop')
    cd /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/format/WebPShop/mac
    /Applications/Xcode_11.3.app/Contents/Developer/usr/bin/ResMerger -dstIs DF /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/Objects/WebPShop.rsrc -o /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/WebPShop.rsrc

ResMergerProduct [...]

However on another machine with Mac OS X 10.14.6 18G2022 and xcodebuild 11.3 11C29 (so almost the same config) but no Internet access, I encounter the following error:

ResMergerCollector /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/WebPShop.rsrc (in target 'WebPShop' from project 'webpshop')
    cd /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/format/WebPShop/mac
    /Applications/Xcode_11.3.app/Contents/Developer/usr/bin/ResMerger -dstIs DF /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/Objects/WebPShop.rsrc -o /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/WebPShop.rsrc
file /Users/kbuilder/webpshop_build/photoshopsdk/pluginsdk/samplecode/Output/Objs/webpshop.build/Debug/WebPShop.build/ResourceManagerResources/WebPShop.rsrc: ### /Applications/Xcode_11.3.app/Contents/Developer/usr/bin/ResMerger - ERROR: errFSBadFSRef
Command ResMergerCollector failed with a nonzero exit code

How can I get any lead for fixing this? Is there any verbose flag to be set for the ResMerger binary to output something useful?