-q tells jam to stop on the first error, instead of building as much things as possible (and still failing in the end, since there was an error)
It is a problem with extended attributes. What filesystem are you building on? When building in Haiku we can use bfs which fully supports them. On Linux, we have to use an emulation layer since the extended attributes support is missing or not good enough on most filesystems. It looks like this emulation layer (which stores the attributes in extra files) has become confused. The error you see does not come from the path that is printed (which is the original file), but from the mirrored directory hierarchy where the attributes are stored.
Read-only file system was user error on my part. I mistakenly thought I had created a folder on the disk when instead I created a node in the root namespace. That’s all sorted now.
My first attempt to boot with a newly compiled kernel stopped without breaking into the kernel debugger. Just hung at the third or fourth icon lighting up. The next time it gave the same error about not finding a boot partition. It send like the kernel is only scanning the internal NVMe internal SSD and not considering any partitions from the USB-C (Thunderbolt) connected SSD, even though an earlier part of the syslog clearly showed that the haiku partition was found. I’ve attached photos of the last bit of the syslog: