Check
a) that nobody changed setting $g_limit_reporters = ON;
b) that view status of your projects is set to "private"
c) the page "Workflow Thresholds" in row "Limit reporter's access to their own issues" where you can override setting $g_limit_reporters

I think (c) resolved the issue. Thanks! ^^,

I have to ask, though:

Where can I find "$g_limit_reporters"?

Also, if I were to set the project to "private", would it not mean the project will not be available to users below developer-level?

Where can I find "$g_limit_reporters"?

the default (OFF) is set in config_defaults_inc.php
if you want to change it, you have to add your setting in config_inc.php

Also, if I were to set the project to "private", would it not mean the project will not be available to users below developer-level?

No, it will be available to administrators, managers of the private project and all other individual users which are assigned.

namao, as this is not a bug or feature request for MantisBT (you are asking for help on how to configure MantisBT) I am resolving this issue as "no change required".
Please use the forums, the help mailing list or IRC to get support on customising and using MantisBT.
http://www.mantisbt.org/support.php