Errors in Firebird.log

Firebird log file (firebid.log) can contain a lot of various messages, here you can find the list of most frequent of them, with the explanation of the error.

INET/inet_error: read errno = 10054

Short: Software caused connection abort.

The disconnect of the client from the server. If error text contains with (Client), it means that the client application lost its connection to the server and wrote down this fact to the log.
If error text contains (Server), it means that server lost the connection to the client and reported it to the firebird.log.
The usual reason of 10054 error is an unstable connection, for example, weak Wi-Fi.
Also, it is possible to see this error if a client application doesn't explicitly close the database connection, i.e., there is no explicit command like "MyDB.Active:=false" on closing the software.

INET/inet_error: read errno = 104

Short: Software caused connection abort.
The same as 10054, but on Linux.

WNET/wnet_error: ReadFile end-of-file errno = 109

In short: Software caused connection abort.

The same as 10054, but this error occurs when client application uses the WNET connection path to the Firebird server instance on Windows, something like this:
This is not recommended, better use TCP/IP connections for network connections (in the format server:path\database.fdb or, on Firebird 3, inet://servername:path\database.fdb), and XNET for local connections (local path on 2.5 and xnet://path\database.fdb).
Consider to disable WNET connections, look here how to disable connection protocols for Firebird on Windows.

INET/inet_error: send errno = 10053 (on Windows)
or INET/inet_error: send errno = 103 (on Linux)

Also means broken connection, but WinSock error is 10053. 

INET/inet_error: connect errno = 10060 (Windows)
or INET/inet_error: connect errno = 10061 (Windows)

In short: 10061 - Connection refused, 10060 - Connection timed out

In general, this error means that it is not possible to establish a connection between the server and client application.

In case of this error with (Client), It means that the client application tried to connect to Firebird through network connection string, but failed, either Firebird server is not running, or access closed by a firewall.

More details about common Winsock errors is here.