I think you might have a firewall. First try using 127.0.0.1 instead of 192.168.4.116

in /electrs/config.toml

~$ curl -sSL "http://mempool.myserver/api/address/1KXLMMcwtB9VsbBXboaV6AxUv7mmomWEeR"
{"address":"1KXLMMcwtB9VsbBXboaV6AxUv7mmomWEeR","chain_stats":{"funded_txo_count":0,"funded_txo_sum":979079,"spent_txo_count":0,"spent_txo_sum":0,"tx_count":1},"mempool_stats":{"funded_txo_count":0,"funded_txo_sum":0,"spent_txo_count":0,"spent_txo_sum":0,"tx_count":0},"electrum":true}

I think it is likely the error is not on the Bitcoin side it is rather on the mempool. To be sure, try to add the debug configurations on my post above, I believe it will give clarity why the mempool is returning HTTP 500 since it will log more output on both electrs and mempool.

Dec 13 14:43:40 [75581] DEBUG: Fetched transaction 1 / 6
Dec 13 14:43:40 [75581] DEBUG: Fetched transaction 2 / 6
Dec 13 14:43:40 [75581] DEBUG: Fetched transaction 3 / 6
Dec 13 14:43:40 [75581] DEBUG: Fetched transaction 4 / 6
Dec 13 14:43:40 [75581] DEBUG: Fetched transaction 5 / 6
Dec 13 14:43:40 [75581] DEBUG: Fetched transaction 6 / 6
Dec 13 14:43:40 [75581] DEBUG: Mempool blocks calculated in 0.001 seconds
Dec 13 14:43:40 [75581] DEBUG: New mempool size: 470 Change: 6
Dec 13 14:43:40 [75581] DEBUG: Mempool updated in 0.01 seconds
Dec 13 14:43:41 [75581] DEBUG: Electrum attempting reconnect...
Dec 13 14:43:41 [75581] ERR: Electrum error: {"errno":-61,"code":"ECONNREFUSED","syscall":"connect","address":"192.168.4.116","port":50001}
Dec 13 14:43:41 [75581] ERR: Electrum error: {"errno":-61,"code":"ECONNREFUSED","syscall":"connect","address":"192.168.4.116","port":50001}
Dec 13 14:43:41 [75581] INFO: Disconnected from Electrum Server at 192.168.4.116:50001

rpcbind:127.0.0.1
rpcbind:192.168.4.116
rpcallowip:192.168.4.122
rpcport:8332

Dec 13 14:45:53 [75763] DEBUG: updateMempoolInfo: connect ECONNREFUSED 192.168.4.116:8332
Dec 13 14:45:53 [75763] WARN: runMainLoop error: connect ECONNREFUSED 192.168.4.116:8332. Retrying in 10 sec.
Dec 13 14:45:53 [75763] DEBUG: {"errno":-61,"code":"ECONNREFUSED","syscall":"connect","address":"192.168.4.116","port":8332}

# bitcoin-cli getaddressinfo 1MDkeWhcTzvnZdqPbJDtPgrLp6QHJeAPNm
error code: -18
error message:
No wallet is loaded. Load a wallet using loadwallet or create a new one with createwallet. (Note: A default wallet is no longer automatically created)

# bitcoin-cli createwallet "testwallet"
{
  "name": "testwallet",
  "warning": ""
}

# bitcoin-cli getaddressinfo 1MDkeWhcTzvnZdqPbJDtPgrLp6QHJeAPNm
{
  "address": "1MDkeWhcTzvnZdqPbJDtPgrLp6QHJeAPNm",
  "scriptPubKey": "76a914ddcbf1131395a90e772396e0e5bad51cc2923c7388ac",
  "ismine": false,
  "solvable": false,
  "iswatchonly": false,
  "isscript": false,
  "iswitness": false,
  "ischange": false,
  "labels": [
  ]
}

Request URL: http://mempool.myserver.com/api/address/1MDkeWhcTzvnZdqPbJDtPgrLp6QHJeAPNm
Request Method: GET
Status Code: 500 Internal Server Error
Remote Address: 127.0.0.1:61274
Referrer Policy: strict-origin-when-cross-origin
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 5
Content-Type: text/html; charset=utf-8
Date: Sun, 12 Dec 2021 00:35:02 GMT
ETag: W/"5-fy9qFc+NorJ+Wkr0e1jnrXHAs9k"
Server: nginx
X-Powered-By: Express
Accept: application/json, text/plain, */*
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: keep-alive
Host: mempool.myserver.com
Referer: http://mempool.myserver.com/address/1MDkeWhcTzvnZdqPbJDtPgrLp6QHJeAPNm
Sec-GPC: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36

{
  "loaded": true,
  "size": 572,
  "bytes": 191953,
  "usage": 1004624,
  "maxmempool": 300000000,
  "mempoolminfee": 0.00001000,
  "minrelaytxfee": 0.00001000,
  "unbroadcastcount": 0
}

185.220.101.178 - - [12/Dec/2021:09:01:06 +1100] "GET /api/tx/dba3a64c1117fb5c67dfc4546b18968ff6f6d716705427c86c6d7cb6d87afb3f HTTP/1.1" 200 801
185.220.101.178 - - [12/Dec/2021:09:01:07 +1100] "GET /api/tx/dba3a64c1117fb5c67dfc4546b18968ff6f6d716705427c86c6d7cb6d87afb3f/outspends HTTP/1.1" 501 15
185.220.101.178 - - [12/Dec/2021:09:01:51 +1100] "GET /api/address/bc1qn75z0k77ywdc47cu9t68xg67rr3g0sw3pm47jv HTTP/1.1" 500 5

In the case of the Bitcoin node, you can use getchaintips RPC and it will tell you the current block height the node has processed. It also has a field for flags and at what block height they are activated; chances are if you aren't anywhere close to the Taproot flag block height then your node is not completely verified.

There is also a friendlier getblockchaininfo RPC that gives you the verification process as a decimal between 0 and 1 ( 1=fully verified ).