OK, this took me a little while to figure out so it might help someone:
If you get the following error from standalone NCBI BLAST+ (blastx v2.2.31 in my case), it means that one of the database files is corrupted.
Fix: just rebuild the database using the makeblastdb command.
Error: NCBI C++ Exception:
T0 "/home/coremake/release_build/build/PrepareRelease_Linux64-Centos_JSID_01_80348_22.214.171.124_9008__PrepareRelease_Linux64-Centos_1433254587/c++/compilers/unix/../../src/serial/objistrasnb.cpp", line 176:
Error: ncbi::CObjectIStreamAsnBinary::UnexpectedTagClassByte() - byte 0: unexpected tag class/constructed: None (0), should be constructed/None (32) ( at Blast-def-line-set)
Just thought I'd leave this here since I didn't find any description of the issue elsewhere.