I just had a look through CipSoft's legal documents, and they do not specify ANYTHING in their legal documents regarding using their client to connect to other servers. So...
Hosting a server is not illegal.
Connecting to a private server with the tibia client is not illegal.
However!
Modified clients is illegal.
This because you are basically taking something which belongs to CipSoft, altering it, and then you redistributing it.
This can be compared to downloading a song made by Britney Spears (Or any other artist), removing her singing and replace it with yourself singing, and then giving out the song and claim its yours.
So you are in the green as long as you do not offer a modified tibia client.
As for donations, those are okay as long as they are defined as "donations" all over the site, and not as "cash-shop items", "Cash items" or anything else implying that the users is BUYING in-game items/services from you.
This is because donations is defined as users willingly giving you their money to support the server, and then perhaps getting something in return for their donation.
However cash shop items is defined as you using CipSoft's software to make a server and then offer buyable services that users BUY from you. Thus you are effectively making money off something CipSoft created.
This is highly illegal.
This can be compared to downloading a copy of Windows 7, cracking it so it doesn't require verification by Microsoft to function (Serial# & WGA), then renaming it to "Doors 7" and selling it for $50.
Microsoft would sue you for millions in no-time.
You should all be happy actually. CipSoft is the most lenient MMORPG developer I've seen when it comes to sueing private servers.
Ragnarok Online and MapleStory servers have had HUGE issues with private server owners being sued by Gravity/Wizet for using their game to host private servers.
TL;DR Version:
Hosting an OpenTibia Server. Okay.
Using the Tibia client to connect to an OpenTibia Server. Okay.
Having "donations". Okay.
Creating a modified Tibia client and giving it to your players. Not Okay.
Having "cash shop items" or "premium services" that players buy. Not Okay.