1. I have problem to open a gallery : images are not loaded or sometimes I get No config.xlm file... Eventually I have to close JB and open it again to open a gallery
That sounds very strange (especially as the problem seems to be intermittent and not consistent).
First of all, if you are not already using the latest version of JuiceboxBuilder-Pro (v1.5.1), then you might like to try upgrading (upgrades are free within the same major version number) to make sure that any bugs from previous versions which have since been fixed are not contributing to your problem.
Please see the Upgrading Juicebox support page for details.
If you have any security software installed, try making an exception for the JuiceboxBuilder-Pro executable file (or temporarily disabling it) to see if this makes a difference. (Maybe it is somehow interfering with the JuiceboxBuilder-Pro processes.)
Maybe you could zip and upload your gallery folder somewhere (your web server or a file sharing service such as Dropbox) and provide a link so that I can try opening your gallery in JuiceboxBuilder-Pro on my own computer to see what happens.
Also, if there is any sequence of steps which you take to reliably reproduce the problem, please let me know so that I can try to replicate the problem on my own system and investigate further.
Thank you.
2. In config.xml I have :
maxImageWidth="1024"
maxImageHeight="768"
largeImageWidth="2048"
largeImageHeight="1536"
smallImageHeight="600"
but no smallImageWidth line... strange... ?
Only non-default values are written to the gallery's XML file (this prevents the need to always have an entry for each and every configuration option) so if you use the default value for smallImageWidth of 800, then there will be no entry for it in the gallery's XML file (and JuiceboxBuilder will use the default value of 800).
If you change a default value, then an entry will be written to the XML file. If you then change the value back to the default value, the entry will remain in the XML file and the default value will be seen. (This would account for smallImageHeight="600" being present in your gallery's XML file, even though 600 is the default value.)
As long as there is a 'small' folder of images in your gallery' 'images' folder and there are corresponding smallImageURL attributes for the <image> tags in your gallery's XML file, then everything is likely to be absolutely fine.
I hope this helps to explain what you are seeing.