unable to install osclass 3.x

[yves31]

Setup a new ubuntu server to test osclass. Download osclass-3.7.3.zip and unzipped to home folder. Changed folder permissions  and used chrome browser to start installation. Went through Welcome screen and pressed Run the Install button. Came to Database Information page and set all required fields. Pressed the Next button and waited a few seconds for database setup.It ended with an error page:
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Osclass » Error

There doesn't seem to be a config.php file. Osclass isn't installed. Need more help? 
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Browser address showed oc-includes/osclass/install.php. If I enter the install.php page in the browser, installation process starts again. The error repeats if I try to install.

The database for osclass was created during the unsuccessful installation process with 38 tables and 371 rows.

Here is information about the system:
Ubuntu 17.04
Apache server 2.4.25
PHP 7.0.15
MySQL 5.7.17

Please help.

[yves31]

I got error.log here:
PHP Fatal error:  Uncaught Error: Call to undefined function mb_strlen() in /www/oc-includes/osclass/helpers/hValidate.php:123\n
Stack trace:\n
#0 /www/oc-includes/osclass/ItemActions.php(100): osc_validate_max('Example Ad', '100')\n
#1 /www/oc-includes/osclass/install-functions.php(505): ItemActions->add()\n
#2 /www/oc-includes/osclass/install-functions.php(426): oc_install_example_data()\n
#3 /www/oc-includes/osclass/install.php(114): oc_install()\n
#4 {main}\n
  thrown in /www/oc-includes/osclass/helpers/hValidate.php on line 123, referer: http://localhost/oc-includes/osclass/install.php

[yves31]

OK
Problem fixed after install php7.0-mbstring on the server.