BSON does not support unsigned values.

[chang-code-hub]

BSON does not support unsigned types, if the number larger than signed type max value, it throws exception:

Value is too large to fit in a signed 32 bit integer. BSON does not support unsigned values.

[cnblogs-dudu]

Update to EnyimMemcachedCore 2.1.0 and use BinaryFormatterTranscoder.

{
  "enyimMemcached": {
    "Transcoder": "BinaryFormatterTranscoder"
  }
}