Attachment upload shows internal server error

[bijumk]

Hello there

I'm using mybb 1.8.5 on a dedicated server. For the last 2 weeks I'm getting "Internal Server Error" when trying to upload pdf files larger than 6 MB

checked php.ini for changes... but it is  good

post_max_size = 50M
upload_max_filesize = 20M
memory_limit = 128M

Settings>> Attachments in Mybb Admin Panel shows maximum size for .pdf is 12 MB.

Please tell me where to look for the cause of the error..

Thanks in advance

Biju

[Matt]

First priority after this should be to upgrade to 1.8.10, 1.8.5 is over a year and a half old.

The best place to look would be the Apache error logs on your server.