i tried to upload simple file as attachment and track down on unix that it was stored in
/var/cache/tomcat9/Catalina/localhost/xwiki/store/file/pgdenik/3/1/3e788f22649d3e3637760f77439996/attachments/4/6/2bb0b019e7cb2184e201fc4b5da8b6/f.igc
it was this issue - I had to explicitly allow tomcat to write to my specific perm directory - which solved it. thanks!