Problem editing CSS

[CPokemon]

Hello! I'm looking a solution to a recent problem. I tried to make a couple of modifications to the css archives of my forum, but every time I want to save the changes, get the following error:

syntax error, unexpected '/' on line 1 in /home/cpokemon/public_html/foro/includes/class_core.php(5559) : eval()'d code
#0 /home/cpokemon/public_html/foro/includes/adminfunctions_template.php(5248) : eval()'d code(52): vB_Template_Runtime::runMaths('/2-1')
#1 /home/cpokemon/public_html/foro/includes/adminfunctions_template.php(5248): eval()
#2 /home/cpokemon/public_html/foro/admincp/template.php(2218): check_template_errors('$final_rendered...')
#3 {main}

Fatal User Error: syntax error, unexpected '/' on line 1 in ..../includes/class_core.php(5559) : eval()'d code in ..../vb/vb.php on line 286

#1 : vbstop() called in ..../includes/class_core.php on line 6034
#2 : vbulletin_error_handler() called in  on line 
#3 : trigger_error() called in ..../vb/vb.php on line 286
#4 : vB::handleException() called in  on line

I recently updated to version 4.2.5 to support PHP 7. I know the risks of that, because it is an alpha version, but don?t know if it is caused by this reason or not.