Some users have seen this message in ST6. ST6 introduced new undo support that allows undo of part operations while working in the assembly.
By default, SE Options, Undo Steps are set to 100. If you have encountered this message the solution has been to adjust this setting down to 50 and this resolved the issue in most cases.
ST6 MP6 has a fix for the issue all together. If you have experienced this message we advise upgrading to MP6. If unable to upgrade to MP6 please adjust your undo steps via SE options to 50.
You can download MP6 from GTAC here: https://download.industrysoftware.automation.siemens.com/solid_edge/ST6/