Try using AES256C or AES2563DES or other protocols listed on https://www.logicmonitor.com/support/getting-started/advanced-logicmonitor-setup/defining-authentication-credentials#snmp (copied below). It seems like using AES256 isn't fully standardized and different vendors implement it in different ways. If your really uses AES128, then I believe that is standard.
- Both AES and DES are supported. AES is the default privacy algorithm.
- AES and AES128 are same protocols and can be used interchangeably because they use the same encryption method – AES128.
- AES192-3DES, AES1923DES, and AES192C are same protocols and can be used interchangeably because they use the same encryption method.
- AES256-3DES, AES2563DES and AES256C are same protocols and can be used interchangeably because they use the same encryption method.
- AES128 and AES256 are supported out-of-the-box.
- DES, 3DES, AES192, and AES256 are also supported.
- AES2563DES and AES1923DES are supported (usually for Cisco devices) starting with EA Collector 28.607 and later.