BuddyPress.org

Concerning your answers In this thread.

Did you install WPMU as a directory or subdomain install? subdomains

Did you follow the detailed instructions in the WPMU’s readme.txt file about subdomain installs (listed under the heading DNS).

Did you upgraded from a previous version of WPMU? If so, from which version? Yes,I upgraded wpmu when I upgraded bp to 1.0 (not sure what previous version of WPMU was)

This could be an issue. You need to follow a specific WPMU upgrade path. If you do not, you can have all sorts of issues. See these links:

https://codex.wordpress.org/Upgrading_WPMU

https://codex.wordpress.org/Upgrading_WordPress_Extended

Do you have any plugins other than BuddyPress installed and activated? 3 plugin widgets of my own that display posts from the database

…AND…

Are you using the standard BuddyPress themes or customized themes? customised themes

To help you figure out what is going on, you need to distill your install down to the lowest common denominator. This means stripping out all extra stuff and focusing on the barebones environment.

So, you need to deactivate ALL plugins except BuddyPress. You need to use the default themes that come with BuddyPress. If the problems go away, then it is related to your custom theme and/or one of more of your plugins.

Here are a few more questions:

1. Which hosting firm are you using?
2. Did you manually install WPMU + BuddyPress or use a one-button installer?
3. There have been a few changes over time to WPMU’s wp-config.php file. If you simply used an older file this could also cause problems. Always verify that the config file you are using is up to date by comapring it to the config file that comes with each, newly downloaded WPMU install package. Did you use the wp-config.php file that came with WPMU 2.7.1?