My main development configuration for fpspreadsheet has been Laz trunk / fpc 2.6.4, but I am also using Laz trunk / fpc trunk intermittently; fpspreadsheet is in a shared folder accessible to both installations. For some time now I am experiencing compilation problems when going from fpc trunk back to fpc 2.6.4. The laz_fpspreadsheet and laz_fpspreadsheet_visual packages alone compile fine without any issues, but when I rebuild the IDE and this process enters compilation of lazarus.exe there occurs an error in one of the fpspreadsheet files saying

"fpsregfileformats.pas(20,32) Fatal: Cannot find fpsReaderWriter used by fpsRegFileFormats, incompatible
ppu=D:\Prog_Lazarus\svn\lazarus-ccr\components\fpspreadsheet\lib\i386-win32\fpsreaderwriter.ppu, package laz_fpspreadsheet"

I know this message from circular unit references, and the first time I saw it I could fix it by restructuring the unit dependencies. At least I thought so, because next time the error came back. Now I am removing both packages, adding them again, deleting unit directories, doing clean builds, recompile both packages in different order, etc - in rather blind fashion because I don't know what is happening. After some time suddenly the compilation passes correctly and everything is working fine again.

Could anybody give me a hint what is going wrong here? Is it really circular unit referencing? And: Is anybody else seeing these issues? Maybe it's just due to my sharing of the same package source folder with two compiler installations.