My nearly 10-year old installation of MLF is starting to do some strange things (can't edit users, for example) and I'd like to be able to do a fresh install then importing a mysqldump file.
I tried doing this on phpmyadmin, and it didn't work. I'd get a blank page, or once it seemed to work but without any theme.
If you say, your installation is nearly 10 years old, what version of MLF are you running? If you didn't update your installation to a recent version over the years, the table structure of your installation is not compatible with the structure of the current version (2.4.24). There was made many changes at the database structure.
In the worst case you have to update your installation step by step to the recent version to get all changes in the structure of the MLF-database (new tables, altered tables). How to do it depends on the starting point/version. That way you would be able to create a database dump in a appropriate format for the current MLF-version.
Afterwards you should be able to create a fresh installation of MLF and to restore the database backup in your new installation.
Beside your problem it would be interesting to know the cause of your initial problem (not being able to alter a user profile). Older versions run on a not well optimised database setup with absent indexes for several columns in several of the tables. I would expect this to be slow in case of many users or many threads and entries but not to be unable to fulfill the tasks.
Trenne niemals Müll, denn er hat nur eine Silbe!