[Lazarus] Compilation issues of fpspreadsheet when changing from fpc-trunk to fpc 2.6.4

[Werner Pamler]

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.