Problem opening .qar archive

I have created a .qar file from a working project in 11.1, which contains an SOPC Builder project. Re-opening the .qar file in 11.1 Quartus seems OK, but whenever I try to run QSys or SOPC Builder from the tools menu, I get this error:

Error: altpll: couldn't read file "../altera_avalon_mega_common/sopc_mwizc.tcl"
(file line 2)
invoked from within
"source "../altera_avalon_mega_common/sopc_mwizc.tcl""
(file "/home/LINX_SCLR/LINX_SCLR_7_2_89_restored/altera_avalon_altpll_hw.tcl" line 57)


Any thoughts?