After update the codeigniter version 2.1.4 to 3.1.3 then show some error
An Error Was Encountered
Unable to load the requested class: Model
Also view some extra error for php version 7.1
A PHP Error was encountered
Severity: 8192
Message: Function mcrypt_get_iv_size() is deprecated
Filename: libraries/Encrypt.php
Line Number: 314
A PHP Error was encountered
Severity: 8192
Message: Function mcrypt_decrypt() is deprecated
Filename: libraries/Encrypt.php
Line Number: 323
A PHP Error was encountered
Severity: 8192
Message: Function mcrypt_get_iv_size() is deprecated
Filename: libraries/Encrypt.php
Line Number: 314
A PHP Error was encountered
Severity: 8192
Message: Function mcrypt_decrypt() is deprecated
Filename: libraries/Encrypt.php
Line Number: 323
May be this post can help you : Call to undefined function mcrypt_decrypt() - even when php5-mcrypt installed decrypt-even-when-php5-mcrypt-installed