why don't people read what was said in other comments before replying
the ONLY problem here is the encoding of the signature that when it generates r and s, r is 32 bytes, s is 31 bytes. it doesn't pad s so it ends up with a 64 (1+32+31) byte signature instead of 65 (1+32+(1+31)).
if you change that, the signature becomes valid. try the following and see for yourself:
IFkS40AxIuRGDN/vnjwavfV6R00h3MKArtDB7tpBbnqIADHOlkhiEJv8JuJTeyVW2Oo23jH9u8fFTqiOms4MM9k=
this is NOT a problem with Electrum, at least not the new versions. i DID test it with my own key and brute forced a signature with a smaller r or s and it DID generate the right signature encoding.
3. the signature I get: IFkS40AxIuRGDN/vnjwavfV6R00h3MKArtDB7tpBbnqIMc6WSGIQm/wm4lN7JVbY6jbeMf27x8VOqI6azgwz2Q==
your signature length is
invalid, it should have been 65 bytes but what you posted here is only 64 bytes which means it is missing 1 byte.
this should not have happened, i can't see any bugs that could lead to this either. are you sure that you have not modified the signature yourself?