I run a regular Apache + mod_wsgi + nginx website on a VDS and encountered a strange problem. The website works fine from my computer and also from our managers' one (who use a different Internet provider). However, some of our website users report to managers that they can't open it in their browser, for a long time period, while at the same moment it worked from my computer and managers'.
The question is: how should I do the diagnostics (if this bug is even my fault) and what bug should I look for if it is impossible for me (even if I wanted to) to go to the problematic user and do the diagnostics from their computer?
UPDATE: Thank you guys for your support! I am going to contact the faulty clients through our managers, do the suggested tests online with them, and then I'll provide additional info if necessary.
UPDATE #2: I managed to contact one of the problematic clients and performed the diagnostics with wireshark. It turned out that the trouble was caused by a bug in the notorious get-iana.sh script in Firehol. As a result, a certain IP address range was incorrectly identified as RESERVED. Everything worked fine... until our local ISP started using those IP addresses for their dynamic IP pool and some website users became locked out. As an aftermath, it seems that I shouldn't have used Firehol in the first place since it is no longer maintained and neither does it support IPv6. Thanks again everyone for your answers.