[DOCS] add note about unicode BOM in password files (#20598)

Co-authored-by: Felix Lange <fjl@twurst.com>

docs/_interface/FAQ.md

@@ -59,3 +59,10 @@ Not only is storing the data very suboptimal, but due to the 200 modification /
 **A:** Unfortunately not. Doing a fast sync on an HDD will take more time than you're willing to wait with the current data schema. Even if you do wait it out, an HDD will not be able to keep up with the read/write requirements of transaction processing on mainnet.
 
 You however should be able to run a light client on an HDD with minimal impact on system resources. If you wish to run a full node however, an SSD is your only option.
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+---
+
+**Q: When I try to use the --password command line flag, I get the error "Could not decrypt key with given passphrase" but the password is correct. Why does this error appear?**
+
+**A:** Especially if the password file was created on Windows, it may have a Byte Order Mark or other special encoding that the go-ethereum client doesn't currently recognize.  You can change this behavior with a PowerShell command like `echo "mypasswordhere" | out-file test.txt -encoding ASCII`.  Additional details and/or any updates on more robust handling are at <https://github.com/ethereum/go-ethereum/issues/19905>.