It seems that one of the projects in your solution or maybe some 3rd party dll has been built with different version of log4net. Either you update references to log4net in all projects or you could add assembly redirection setting to the web.config
Put below tags in Web.config, anywhere under configuration element.
<runtime>
<assemblybinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentassembly>
<assemblyidentity name="log4net">
publicKeyToken="1b44e1d426115821"
culture="neutral" />
<bindingredirect oldversion="1.2.10.0">
newVersion="1.2.13.0"/>
</bindingredirect></assemblyidentity></dependentassembly>
</assemblybinding>
</runtime>