Migration Error from NIOS 11.2 to NIOS 12.1

After rebuilding my project after a long pause I discovered a building error in NIOS that seems to be related to the update from NIOS 11.2 to NIOS 12.1:

Description    Resource    Path    Location    Type
Symbol 'EINVAL' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 103    Semantic Error
Symbol 'ESRCH' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 111    Semantic Error
Symbol 'SIGABRT' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 63    Semantic Error
Symbol 'SIGALRM' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 64    Semantic Error
Symbol 'SIGBUS' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 74    Semantic Error
Symbol 'SIGCHLD' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 90    Semantic Error
Symbol 'SIGFPE' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 65    Semantic Error
Symbol 'SIGILL' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 66    Semantic Error
Symbol 'SIGKILL' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 67    Semantic Error
Symbol 'SIGPIPE' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 68    Semantic Error
Symbol 'SIGPOLL' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 75    Semantic Error
Symbol 'SIGPROF' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 76    Semantic Error
Symbol 'SIGQUIT' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 69    Semantic Error
Symbol 'SIGSEGV' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 70    Semantic Error
Symbol 'SIGSYS' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 77    Semantic Error
Symbol 'SIGTERM' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 71    Semantic Error
Symbol 'SIGTRAP' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 78    Semantic Error
Symbol 'SIGURG' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 91    Semantic Error
Symbol 'SIGUSR1' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 72    Semantic Error
Symbol 'SIGUSR2' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 73    Semantic Error
Symbol 'SIGVTALRM' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 79    Semantic Error
Symbol 'SIGXCPU' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 80    Semantic Error
Symbol 'SIGXFSZ' could not be resolved    alt_kill.c    /converter_bsp/HAL/src    line 81    Semantic Error
Invalid  project path: Include path not found  (C:\APPS\Altera11\quartus\bin\cygwin\lib\gcc\i686-pc-cygwin\3.4.4\include).     converter        pathentry    Path Entry Problem
Invalid project  path: Include path not found  (C:\APPS\Altera11\quartus\bin\cygwin\lib\gcc\i686-pc-cygwin\3.4.4\include\c++).     converter        pathentry    Path Entry Problem
Invalid project  path: Include path not found  (C:\APPS\Altera11\quartus\bin\cygwin\lib\gcc\i686-pc-cygwin\3.4.4\include\c++\backward).     converter        pathentry    Path Entry Problem
Invalid project  path: Include path not found  (C:\APPS\Altera11\quartus\bin\cygwin\lib\gcc\i686-pc-cygwin\3.4.4\include\c++\i686-pc-cygwin).     converter        pathentry    Path Entry Problem
Invalid project  path: Include path not found  (C:\APPS\Altera11\quartus\bin\cygwin\usr\include).    converter         pathentry    Path Entry Problem
Invalid project path: Include path  not found (C:\APPS\Altera11\quartus\bin\cygwin\usr\include\w32api).     converter        pathentry    Path Entry Problem
No break at the end of case    alt_kill.c    /converter_bsp/HAL/src    line 89    Code Analysis Problem
No break at the end of case    alt_kill.c    /converter_bsp/HAL/src    line 103    Code Analysis Problem
Unused declaration of function '_do_ctors'    alt_main.c    /converter_bsp/HAL/src    line 54    Code Analysis Problem
Unused declaration of function '_do_dtors'    alt_main.c    /converter_bsp/HAL/src    line 55    Code Analysis Problem

The reason why I think that these errors are related to the software updates are that the invalid project paths are pointing to the previous location - ".../Altera11/...". (Why is still this previous path used?)

All errors occur in the BSP (Board Support Package) which is automatically generated.

Since this error did not occur previously, an include file (probably the standard library ?) seems to be missing.

Before building, I ran the whole workflow process - generating in Qsys, building in Quartus and updating the BSP in NIOS.

All my attempts to solve this issue failed so far.