I don't know if this is really a bug b'cos it happens rather randomly and I only came across it twice and was unable to reproduce it on purpose:

when installing or upgrading modules i sometimes come across errors statings that there is a 'Failed open file' and that XOOPS was 'unable to write to main menu' and the module's admin icon didnt show up (at the left of admin page).

Luckily it can be fixed by deleting the adminmenu.php under the cache directory and have XOOPS regenerate the menu.

does anyone have the same problem?

can't say i've ever had that problem, and with it being random i dunno what to suggest other than a server issue, or somethin..

i have random cases in IE where it decides to view the page but refuses to read the CSS lol so i get white pages with absolutely no formatting whatsoever, it's damn annoying and a hit of the refresh button will sometimes work.. technology huh..