I have chossen either /droot/sitemap and /droot/forum/sitemap, both set to the user this domain belongs to and both set to 777,
but the script wont write my sitemap-files.
Please try creating a folder called store_sitemap in the folder where your forum files reside, CHMOD it to 0777 and set the sitemap path to ./store_sitemap and see if that works for you.
Path issues would indeed prevent the sitemap from building correctly.
But tell me please, it is possible that the failure comes from session-management and not from rights to write?
Is it possible that another addon will disturb the script?
Maybe a changed table?
I hope you will find time to find this error in your script and solve it till the next update.
The problem was/is that the value dbtech_dbseo_sitemapsession is written in table adminutils, but removesession.php asks for sitemapsession and not for dbtech_dbseo_sitemapsession.