Hi Roman,
Firstly, check your downloaded files. Most of cases these error may occurs due to incomplete downloads. You can compare the size of the files you have, than with download center.
Secondly, network shares are always prone to have access rights issues. Though you could see the folders at "\\192.168.20.44\SAPInstallation" with your current user (<hostname>\Administrator) , at the time of installation SAP use to create number of local users like <SID>adm and SAPservice<SID>adm and would try to access the installation files with these users. Hence it wont have enough rights to access them.
It is always safe to have the installation files at the local disk of the server on which you're trying to install. This will also speed up your installation process as the access to network share would add time load to the installation.
Cheers!!
Mahendra varman