Electrum is only for bitcoin related threads as far as I seen,so will it okay to post BTX's electrum query there?
Since you already mention your issue above I would like to suggest a solution to fix your issue.
Since your problem is when sending your BTX coin from Electrum segwit to Bit-z legacy address you are getting an invalid address which I think there is incompatibility issue on Electrum part.
Let me know if what is your BTX Electrum version? There is a 2 BTX electrum version the one is using the wrong BIP32 derivation path and another one is fix including the other error that you will get from old version of Electrum.
Fixed BIP32_Prime.
Security Fix for Server Error Message.
Now using the correct BIP32 derivation path.
So you might be using an Electrum wallet which is old so check this version 3.2.3-btx with BIP32_PRIME fix
Take note make sure that you already have backup seeds or private keys of your wallet before you upgrade it.
*Another solution.
I think you can also make a new wallet from Electrum and make a legacy address and transfer your BTX from segwit wallet to legacy.
*Another possible solution try to install a coinomi wallet in your android device and add bitcore coin and send your BTX from segwit wallet to Coinomi wallet.
*Another solution.
Try other exchange sites you can find them from here https://www.coingecko.com/en/coins/bitcore/trading_exchanges#panel
Exrate exchange is one of my favorite site of shitcoin and I haven't experienced any issue yet.