The assert seems to be triggered by Messages, because the German translation of the message "Paste" ("Einfügen") contains an umlaut (see item #5 in the backtrace).
Jürgen Assertion triggered in const lyx::docstring lyx::from_ascii(const char*) by failing check "static_cast<unsigned char>(*c) < 0x80" in file docstring.C:25 We got a translation LyX: Versuche, das Dokument /home/juergen/test/spellcheck.lyx zu speichern... /home/juergen/test/spellcheck.lyx.emergency We got a translation Speichern scheint gelungen zu sein. Glück gehabt! Program received signal SIGABRT, Aborted. [Switching to Thread -1222023504 (LWP 24893)] 0xffffe410 in __kernel_vsyscall () (gdb) bt #0 0xffffe410 in __kernel_vsyscall () #1 0xb72c67d0 in raise () from /lib/libc.so.6 #2 0xb72c7ea3 in abort () from /lib/libc.so.6 #3 0x08540d53 in lyx::support::abort () at abort.C:22 #4 0x080bff78 in boost::assertion_failed ( expr=0x862e56c "static_cast<unsigned char>(*c) < 0x80", function=0x862e720 "const lyx::docstring lyx::from_ascii(const char*)", file=0x862e4bb "docstring.C", line=25) at boost.C:57 #5 0x08543904 in lyx::from_ascii (ascii=0x8942ea4 "Einfügen") at docstring.C:25 #6 0x081d14d1 in Messages::Pimpl::get (this=0x8852470, [EMAIL PROTECTED]) at messages.C:200 #7 0x081d0aeb in Messages::get (this=0x87332f0, [EMAIL PROTECTED]) at messages.C:252 #8 0x08125eda in _ ([EMAIL PROTECTED]) at gettext.C:43 #9 0x0817fd71 in LyXFunc::dispatch (this=0x889bdd0, [EMAIL PROTECTED]) at lyxfunc.C:1619