R – Can add the custom webpart but then cannot modify any webpart on page

I`v made my webpart that works well (I can add it to page and use it). However if i want to move it to different zone or modify ANY other webpart on that page i get an exception.

Any ideas on what could be wrong?

Maybe it has to do something with that my web part has a null Zone variable? However webpart displays well on page. Details below.

Edit: Ahh, no, I see that other (original ones) web parts have Zone variable also as null. Anyway i have no idea how to even digg and find out the problem.

Exception occurred. (Exception from HRESULT: 0x80020009 (DISP_E_EXCEPTION)) 
Exception Details: Microsoft.SharePoint.WebPartPages.WebPartPageUserException: Exception occurred. (Exception from HRESULT: 0x80020009 (DISP_E_EXCEPTION))

Stack Trace:     

[WebPartPageUserException: Exception occurred. (Exception from HRESULT: 0x80020009 (DISP_E_EXCEPTION))]
   Microsoft.SharePoint.WebPartPages.SPWebPartManager.SaveSomeChanges(SPLayoutProperties layoutProperties) +897
   Microsoft.SharePoint.WebPartPages.WebPart.SaveSomeChanges(SPLayoutProperties layoutProperties) +52
   Microsoft.SharePoint.WebPartPages.SPWebPartManager.WPSaveSomeChanges(SPLayoutProperties layoutProperties) +83
   Microsoft.SharePoint.WebPartPages.SPWebPartManager.ApplyChangeList(String changeString) +865
   Microsoft.SharePoint.WebPartPages.SPWebPartManager.OnLoad(EventArgs e) +398
   System.Web.UI.Control.LoadRecursive() +50
   System.Web.UI.Control.LoadRecursive() +141
   System.Web.UI.Control.LoadRecursive() +141
   System.Web.UI.Control.LoadRecursive() +141
   System.Web.UI.Control.LoadRecursive() +141
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627