How to deal with errors with multisite in Bitrix?

Question

1C-Bitrix

The bottom line is this, I have a bitrix acc, where there are already 2 sites, I made links from the first site, but there is no admin panel on the site (which is on top) you can go to the admin panel itself, but almost everywhere it was written "Script error" I turned on show mistakes and I have such a f*ck

spoiler

[Error] 
Call to undefined function mb_orig_strpos() (0)
/home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:320
#0: CSecurityXSSDetect::fastStrpos(string, string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:190
#1: CSecurityXSSDetect->findInArray(string, array)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:221
#2: CSecurityXSSDetect->isDangerBody(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:237
#3: CSecurityXSSDetect->getFilteredScriptBody(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:262
#4: CSecurityXSSDetect->getFilteredScript(array)
  
#5: preg_replace_callback(string, array, string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:274
#6: CSecurityXSSDetect->filter(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:72
#7: CSecurityXSSDetect->process(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/security/classes/general/post_filter.php:55
#8: CSecurityXSSDetect::OnEndBufferContent(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/main/classes/general/module.php:490
#9: ExecuteModuleEventEx(array, array)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/main/classes/general/main.php:3191
#10: CAllMain->EndBufferContent(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/main/classes/general/main.php:3139
#11: CAllMain->EndBufferContentMan()
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/main/include/epilog_admin_after.php:24
#12: require(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/modules/main/include/epilog_admin.php:5
#13: require(string)
  /home/e/equiliw5/lpcc.ru/public_html/bitrix/components/bitrix/desktop/admin_settings.php:185

I made links through a script on the Bitrix website, I just don’t understand what’s wrong ... If you know the option, please chew it like a child, I still don’t know what Bitrix is, so I’m asking