Please provide version number and host.
See post https://forums.b2evolution.net/urls-in-excerpt-stopped-post-from-being-saved about mod_security issue with host.
There are other similar posts I suggest you search for them, read them and then you may want to contact you host

So you seem to say that the problem is with my hosting provider?

Hello,

The following frequent issues seem to apply to your post: (This is a semi-automated response):

• You did not provide the Name / URL of your webhost (as requested for support)
• See: https://b2evolution.net/man/mod_security

Webhost is namecheap

Thanks. You need to contact their support and ask them to fix the offending mod_security rule they have in place.

Thank you for your support, i contacted the hosting provider and the problem is solved now.
Thank you again for the wonderful software... Best Regards